It's like a new toy. Once you know about it, you go all crazy because it's cool and fun and omg. Then you get a bit of normalcy back, and then it comes back when you actually get to play with it.
As for Killzone2: the past couple of weeks have been a firefight between pro and against, especially when the reviews were out and the demo too.
Sony haven't done anything wrong with lifting the embargo that soon. The people on fora and the like already know whether they want this title or not, and won't be convinced otherwise most likely, so they are not influenced by the month.
Now all they need to do is to hype the title for the general audience, and that's what should be coming in the next few days (hyping the crowd). And for that, they might have wanted to get some positive reviews that they could use.