I heard Witcher 2 has Steam Achievements. Is that true?

If yes:
Do we get a Steam key on purchase on GOG?

The GOG version is DRM-free. So no serials to register with Steam.

It only has Steam achievements if you buy the Steam version. The GOG version is DRM free and a Steam key is not part of the bonuses so I wouldn't count on it.

Well, the Humble Bundles are also DRM free and have Steam keys, so... :D

Come on, I don't have to buy it twice, or do I?

You don't have to buy it twice. Buy it from Steam if you care about achievements or buy it from GOG if you care about no DRM and their exclusive pre-order bonuses.
Valve gave Wolfire a bunch of Steam keys for free probably because the HIB was for charity and the keys aren't unlimited, they could have run out of keys if too many people bought the bundle. If CD Projekt were to announce Steam keys for purchasing the game from GOG.com Valve will most likely want a cut of the profits.

Why would you buy it twice? Even if you did you still wouldn't have a DRM-free version with achievements so I don't see the point.

They could make a deal where you send a proof of purchase on GOG to Valve or something. It would be possible with credit card, if you ever bought from Steam. ^^

And by the way, you get the preorder bonus without actually buying it from GOG later.

You only get a few pre-order bonuses right away. Most of the bonuses are only if you pay for the game, check the game page there are 3 titles: "Exclusive GOG.com content available immediately", "Exclusive GOG.com content , available on release" and "Digital Premium content, available on release". Obviously the content available on release is only for paying customers.
If you buy from GOG.com you are supporting the developers and it wouldn't be fair if Valve were to get a cut of the profits from GOG.com. You won't get a Steam key if you buy from retail either because Witcher 2 isn't an Steamworks game.

Totally. :P

The only game I think I've ever paid attention to achievements on is Fallout 3. Even that was more as an excuse to keep playing it.

more than achievements it would be better to buy the steam version if it had cloud support it least you dont have to worry about the save files

Why do you need achievements in an RPG game ? Especially Steam achievements .

in my days, games were made out of wood and had no achievments! XD

Considering to buy the game for that abysmal Steam service just for the achievements is... wow. And Valve even did it wrong when they copied Xbox/Games for Windows Live and forgot about meta-game stuff like the gamer score.

It's interesting that people are claiming that Steam keys cost the developer money when all the evidence on that matter I've seen suggested the inverse is true.