Ken posted this over on Quarter to Three, I'm re-posting it here because I agree with this 100% and think it deserves a wider audience, even though it is mostly common sense:
If you're going to buy the game this week, I highly recommend you stay away from any forums that might have spoilers until you play the game through. Our goal in making the game was to thrill and, often, surprise you.
Please don't ruin other people's experience by revealing secrets in unmarked threads, and if you want to enjoy the game to its fullest, stay away from any threads that might ruin the fun for you. With a game like BioShock, it will really make a difference.