Gears 2 won't even be exclusive, just like gears one wasn't. It will be out for the PC in early to mid 09 with better graphics, more levels the 360 couldn't handle and WAY better controls just like with Gears of War one.
360 exclusivity is a myth if you have a PC thats worth more then 600 dollars (see the how to build a computer better then a console for 500 dollars elsewhere on the forum).
Most of the PS3 big hitters will remain just that, PS3 games. You don't see a lot of playstations best sinking into the PC market in any generation while the Xbox or 360 to PC conversions are almost guaranteed.
As a quick caveat, its guaranteed if they're good. Some games like Kameo and Lost Odyssey aren't worth converting to the PC/PS3 due to lack of goodness. Those that are (mass Effect, Gears of War, Bioshock ect.) make the trip off the 360 to other superior gaming grounds. I think Halo 3 is the only good game for the 360 (as in high ratings) that hasn't made the jump to the PC or PS3, and given Halo 1 and 2s presence on the PC I would bet PC Halo 3 is a forgone conclusion.
PSN ID: ChosenOne feel free to add me