I agree that if I was microsoft I'd definitely want to get bioshock and its sequels "exclusive for ever" for my platforms,
but as a gamer I prefer that everybody gets to play this.
That's kinda the point though.
Get enough of these compelling games exclusive on one platform and get the price of entry low enough, and these people that would otherwise not be interested in an xb360, are new owners.
see: ps2 sales
Sony, MS and Nintendo all are shooting for the same thing:
Create a gaming platform compelling enough to woo consumers.
If most games will be multiplat (a given) then the choice of console really boils down to price, exclusive games, and features. (the order of priority will be different for everyone)
MS really should shoot for locking down this game/franchise and the studio behind it (2k Boston?).
Their internal studios aren't exactly world beaters and when compared to a Sony or Nintendo, they are not only out-manned, they are out-gunned. Adding a dev of this calibre would do wonders for them.
Of course if this doesn't happen, I'll be happy to see this title get ported to ps3. You ps3-only guys are in for a treat! But honestly, at this point I can't see a reason you ps3-only guys would still be "ps3-only" with such a lineup coming to xb360 this year and a pretty cheap price. Add the 65nm shrink and better cooling/extended warranty, added hdmi and the rumored 2 game bundle in a few months and I'd say it's time to jump in.