I'm not terribly impressed with Valves programming in general, so my guess would be that they simply forgot about it during development. The thing with mirrors is that they introduce a second rendering queue into the process. Basically, you turn all objects around, render that, then render the normal level over it with the mirror surface being transparent (or you can do it with framebuffer textures, but that doesn't work too well on PCs as framebuffer access is usually pretty slow on anything before the current generation of graphics cards). Whichever way you chose, you have to reininitialize the engine to the new view point and depending on the kind of optimizations you employ (specifically, how aggressively you generate nodes in the BSP tree), the reinitialization can take a while and in the worst case flush out objects that are needed for the current view, which would then have to be reloaded. If you didn't think about that when you started doing the optimizations, it can be pretty hard to add them lateron.

Well... But don't water reflections do pretty much exactly that?

Depends on what kind of reflection you're talking about. First there's the simple "reflection" that doesn't really reflect the world, but a prerendered texture instead.
Then there are real reflections which are usually done using framebuffer access as it allows refraction as well. If they used that then yes, they could have used the same code for mirrors.

That's fairly much what I assume, but sometimes I jump to extremes to make a point. :D
Yes, I like to play the devils advocate.

What a moronic solution. Clipping/collision detection issues are ridiculously easy to fix (even if all they do is make the surfaces solid on both sides, which is how many games handle all surfaces by default), and with a hard drive in every PS3 it's not like there are any technical barriers to releasing a patch.
Such obviously exploitable gaps are unacceptable in a game with this sort of budget. I can think of a huge number of games I've played where such gaps are very rare or simply non-existent (especially in the case of multiplayer games), and many of them wouldn't have had such a huge budget.
I also find it very hard to believe that this was not discovered during beta testing. Experienced beta testers would know the most common places clipping gaps occur and would do their best to glitch through them, and there's also a reasonable chance of testers accidentally doing so while trying to climb up parts of the scenery you're not meant to be able to pass.

Well since we're talking about source and their real-time water reflections...

It's been a long time since I played any source game and I simply didn't remember that. As I said, if the reflections are real, they could have used the same code for mirrors.

It happens. As long as you patch it this is fine by me. But apparently, many developers now prefer banning to actually fixing their games. Point in case: CoD5. There are many, many spots where you can use the faulty collision detection to catapult you up. Just jump aginst some entities and quickly duck and jump again and you'll be lifted above the entity and often to the roof. Or you can go under the map. Are this things known? They sure are, but no patch in sight.

Heh had to look it up and see if the community came up with something yet for this and they did. Its not natively supported but it is creative. Here they are using a preset for scope reflections to simulate a mirror effect:
http://www.youtube.com/watch?v=-bTHlvXQ1P8
All though inverted. You can also see a glimpse of the in-game model for freeman in there and why dynamic reflection on him wouldn't be the best idea.

As you say, it's not, in fact, mirrored at all. I'm guessing this is the aforementioned "tv screen and camera solution", which is not a mirror, and can't be mistaken for one.

Well glitching in games has been around as long as multiplayer, it sucks and they should fix it but if they don't, it's a natural selection thing, people will move to games that work better. At least that's the theory, realistically they'll probably just move to the next glitchy game, spend months bitching about a lack of patch and then repeat.
I've played Gears Of War 2 enough to have seen at least 2 glitches, the "invisible shield whilst using a normal gun" one and the "smoke grenade blowing someone out of the map" one. The latter was most annoying as it was in a game of submission and the dipshits who did it blew the hostage you have to capture out of the map so noone could win.

Walkthroughs don't count as cheating much. I think anyway. But my question is this: If say you have a buddy, and you're stuck on a part of a game. And your friend who is not involved in playing the game says something like "Oh, try doing [such and such]", and it is the solution. Does that count as cheating because he isn't playing the game?
Oh, and Weclock, Hong Kong pride!

I know, it makes me feel warm and fuzzy about myself.

Haha, I do stuff like this all the time. Most recently I was playing Puzzle Quest while a friend watched over my shoulders, pointing out good moves and trying to convince me to use certain spells.

I think walkthroughs are KIND of cheating but in a single player game cheating isn't really relevant, it's only hurting yourself. Hints I like, "The solution involves the manual of style" is good but "GIVE stylish confetti to HEAVILY ARMED CLOWN" is a bit too hand-holdy
Damn, lucky they weren't talking about gay porn...