Since the game doesn't have a "clock" keeping up with time, SC players have gotten used to using their supply number as a measure of time for when you have to build stuff.
So 9/10 Overlord means that by the time you're on your ninth unit, before you hit your tenth, spawn an overlord.

Patch 12 is out. The changes to game mechanics are as follows:
I completely agree with the change to forge. This will help make the cannon rush slightly less viable as the other races effectively get 10 more seconds to prepare / get defense out to kill an early probe.
On the flip side, the mothership vortex just got more beastly.
I also completely agree with the changes to the thor. It was too massive, and because of that had an effective range of one less than it's stated range, since the effective range is taken from the edge of that huge hitbox. Now it's a bit smaller, and this should help with it's sometimes whacky pathfinding too.
I've no idea why they've decided to up the supply cost of the roach to 2 though.

Blizzard does post sometimes explaining why they did the changes they did. They considered the roach too good to be just 1 supply and it turned into a roach fest when playing zerg vs zerg, so with this change they hope to see some unit mixes in that match up.

I like how you can't dick around even a little in this game, make one mistake and you're fucked. Forgot to build that 9th drone 2 seconds ago? Fucked.

Thats the old patch changelog, but anyway...
Ten seconds is too much. Five would have been better. But I've been successful with this rush strategy, so I can't really complain much.
And I like the roach supply nerf. Now at least zerg players will play with other stuff, other than making only roaches (happened a lot before the patch).

To clarify, when I say Extractor Trick, I mean doing the Zerg-exclusive move of making an Extractor on 10/10 food (when you're supply capped), morphing and Extractor (using up a Drone) which brings you down to 9/10 food, creating a Drone (back to 10/10) and then cancelling the Extractor, getting your Drone back (putting you at 11/10, ABOVE the Supply Cap). The jury's out on whether 9 Overlord or 10 Overlord with Extractor Trick gives you better economy, but the difference is marginal enough for no one to agree :)
Personally, I just like the feel of doing the early scout with the Extractor canceling Drone when I'm doing the 14 Hatch build.
The problem with the Roach nerf is that Zerg players don't HAVE anything else to build. Roaches are our only good meat shield for the Hydras. Of course, you can still build them (and it does work out pretty well late game as long as you have enough expansions up) but in early to mid game they just don't cut it any more. But yeah, with Zerglings being completely atrocious against any mid/late-game army and Banelings only being useful against certain builds, I really wish there were more tanking/zoning units for the Zerg. Perhaps tier 2 Lurkers would be the perfect addition at this point? ;)
For the record, Zerg has only 9 different combat units while the other races have 12 and 13 combat units respectively. I really wish there were more options. Nydus sounds good in theory but if you get your army split up while Nydusing, it WILL cost you the game instantly. What I do wish to see from Zerg players is more Ventral Sacs (drop) play. Baneling carpet bombing seems pretty strong.

It appears that the beta will undergo an intermission in two week's time. I say intermission since there's plans for the beta to resume in the run up to launch. But still, get yer Zerg rushes in now.

Yup, if I remember correctly, there was a blue post explaining that even they felt that the zergs were short on units.
Maybe an unit or two will make it into the game before the release? Afaik, the zergs in Brood War had more units. Maybe some could be transfered here?

Its a bit late in the game to be adding entirely new units and trying to balance them in if blizzard does that the games almost gaurenteed to be pushed back from its july release date.

Then I bet you hate burrowed Banelings as well? They do pretty much the same thing minus the zoning. Just throw out a Raven and you'll be fine. Either that or build turrets along with your push.
And yeah, Browder pretty much confirmed there won't be any new units added until the expansions in the latest batch of interviews, which I really think is a shame. Oh well, all the more reason to look forward to Heart of the Swarm :)

This sheds some more light on why zerg have been repeatedly nerfed.
http://www.mymym.com/en/news/18583.html
If the Koreans are so heavily favouring zerg, there must be something up with the balance. After all, game balance shouldn't be discussed by the EU/US platinum players. The EU/US platinum players would have to work hard to stay in the bronze leagues in Korea.
Just like BW, how SCII will ultimately be played will be dictated by the Koreans.

Actually, that is not true, for the most part. While asia won 4-0 in a recent team tournament vs EU, it was far from being a steamroll. The reason korea is so dominant in SCBW is because they live in their professional Team House where they practice 12+ hours a day (minimum requirement). The korean esports organization 'Kespa' is trying very hard to make as much money as possible from SCBW, and they currently consider SC2 (and especially Blizzard itself) a huge threat, which has actually led to them banning players associated with Kespa (all of them) from participating in Blizzard sponsored events, and they also allegedly pressured the government into giving SC2 an Adults Only (18+) rating in Korea which would prohibit minors (which a lot of players are) from buying the game.
What this means is that there are very few professional Korean SC2 players. That's probably how it's going to stay until Blizzard and Kespa can work out their differences. It certainly won't be as big as SCBW is in Korea until then.
Now, when it comes to WC3, Koreans and Europeans were on par with eachother, with the Euro players being horrible and Undead and Night Elf (which the Koreans were great with, especially Night Elf) while Koreans were horrible with Human (which was considered completely overpowered in Europe).
The lesson to learn here is that perhaps there is no need to nerf Zerg, perhaps it's just that Korean players are better at playing Zerg, simple as that. Then again, it is a beta so I'm all for Blizzard experimenting around with pretty much anything trying to find what works and what doesn't. I just hope they don't get too nerf-happy overall, Brood War was a game where completely overpowered imbalanced units simply countered eachother by being even more overpowered, which caused the game to be extremely explosive and balanced at the same time. A game that's been nerfed too hard for the sake of so called "balance" will just end up feeling powerless and not very fun to play.
Sometimes, it's just up to the players finding the right tricks to balance the game out, without any patching.

New Patch Notes:
StarCraft II Beta -- Patch 13 (version 0.15.0.15449)
General
Map Publishing is now enabled: Using the map editor, you can upload your custom maps to share with the Battle.net community.
Facebook feature is integrated: Here’s a quick way to expand your social network by seeing who among your existing Facebook friends also has a Battle.net account.
3v3 and 4v4 formats are now enabled.
Numerous updates have been made to the Leagues & Ladders system:
Removed Copper League and added Diamond League above Platinum League.
Player ratings start at 0, rather than 1000.
No longer displays loading screen odds in placement or practice league matches.
Matchmaking system logic updated.
UDP is enabled to help improve game performance.
Numerous performance and stability improvements.
Balance Changes
PROTOSS
Sentry
Force Fields can now be destroyed by Massive ground units walking over them.
Void Ray
Range decreased from 7 to 6.
Warp Gate
Subgroup selection priority changed from 2 to 3 so that it takes priority over Gateways when selected.
TERRAN
Marine
Stimpack research cost decreased from 150/150 to 100/100.
Combat Shield research cost decreased from 150/150 to 100/100.
ZERG
Infestor
Infested Terran spell removed.
Frenzy spell added:
Costs 25 energy.
Targets a single biological unit which deals 25% more damage and is immune to snare, stuns, and mind control for 30 seconds.
Overseer
Contaminate spell added:
Costs 75 energy.
Targets a single enemy structure which cannot train units or research upgrades for 30 seconds.
Infested Terran spell added:
Costs 125 energy.
Infested Terrans have the same stats as those previously created by the Infestor and are placed directly under the Overseer when spawned.
Ultralisk
Life decreased from 600 to 450.
Damage changed from 25 to 15 (+25 Armored).
Damage versus structures increased from 60 to 75.
Hotkey Changes (English Only):
Zerg Set Worker Rally Point changed from R to G.
Zerg Spore Crawler changed from W to A to avoid conflict with the Select All Warp Gates hotkey.
Battle.net Interface
Revamped summary pages for player Profiles and Leagues & Ladders.
Added a Help system with tech trees and other tips and tricks.
Removed identifier from the character naming process and added the ability to refer friends for invitation into your party or lobby.
Updated the Battle.net user interface to consistently use a nested menu system.
Added in-game blocking and player muting.
---------------------------------------------
Also: The new StarCraft II publishing system on Battle.net allows you to publish your maps and mods directly to the Battle.net service, where they are permanently stored online and made available for others to play within the game. If you want to play your map online with other players, or if you want others to be able to use your mod in their maps, they need to be posted using the publishing system. Below you'll find details and information on how to begin using the publishing system, and some of the nuances to help you on your way.

So the Ultralisk is now only good for destroying buildings?