Oh this has to hurt -- sure-blockbuster Fallout 3 has been leaked very early, and is currently being pirated across many P2P clients.
Before you get on about how piracy is killing the PC, and piracy is the reason why developers don't bless us with games, you should know that it is the Xbox 360 version of the game that is making its rounds.
It seems highly likely that it was a early review copy of the game, given to the press, that has been leaked. The game only went gold yesterday, after all. Hopefully Bethesda will track be able to track down game-leaker and fine him, or punch him with a fatman fist, or what not. A studio can probably handle 0-day piracy of the game they have spent the last 2 years working on, but when the game is leaked 20 days before release, you have to imagine that that will hit them in the pocket books.
Hmm... could more Xbox 360 DRM be coming after this boondoggle? I hope not.