@twesterm - So your argument is that unless the PC version of a game either comes out first or can be modded, the console version will almost always out-sell it? But with Bioshock, note that we're not talking about a game that has the console version coming out well before the PC version. They're both being released simultaneously, so neither version has the advantage you're talking about. No one will buy the console version just because they don't want to wait for the PC version. They don't have to wait - they can choose now.
As for modding, Bioshock runs off the Unreal Engine, one of the most popular and versatile engines as far as modding (you even point out UT3). I see no reason why Irrational wouldn't support modding for Bioshock (they have a history of rewarding their fans this way, like the multiplayer patch for SS2 which they made solely out of the goodness of their hearts), and I see no reason why fans would not take it up with a passion. I think you severely underestimate the cult-following that a game labeled "the spiritual successor to System Shock 2" already has. Not will have, but has now. And this should only grow once the game actually comes out.
I never guaranteed that the PC version will outsell the 360 version, but you are, in fact, guaranteeing the opposite...
That said, the zinger: Oblivion's PC and 360 versions were actually released on the same day. You may be right in the end, but I wouldn't be as sure as you are about it.
"Whenever you find a man who says he doesn't believe in a real Right and Wrong, you will find the same man going back on this a moment later." -C.S. Lewis
"We all make choices... but in the end, our choices... make us." -Andrew Ryan, Bioshock
Prediction: Wii passes 360 in US between July - September 2008. (Wii supply will be the issue to watch, and barring any freak incidents between now and then as well.) - 6/5/08; Wow, came true even earlier. Wii is a monster.