Hmm, let's review.
PC
Left 4 Dead (coop + zombies > all)
Combat Mission - Shock Force (though I may wait on this, since I'm *years* away from finishing the current batch in the series)
Airborne Assault - Battles from the Bulge (same, though given the number of scenarios, I may get this one sooner)
NWN2 Xpac (this is almost by default - I'm not even sure if I'm supporting the game 'just because')
Close Combat rereleases (3 is out, 2/4/5 are not)
360
Too Human (coop action RPG ^__^)
Halo 3 (coop + Legendary difficulty = hehehhehehehhehe, though I'll delay if I haven't finished Halo 1/2 by then)
Mercenaries 2 (coop + sandbox + destroying everything + burning down jungles)
Army of Two (coop, though the devs' lack of credentials makes me at least a bit skeptical)
Eternal Sonata (JRPG with a fun battle system - great demo; coop makes this a day1 purchase)
Blue Dragon (another JRPG, though lack of coop makes this NOT a day1 purchase)
Mass Effect (bargain bin at best, if even that - coop would have made this a day1 as well. When will devs learn?)
Ace Combat (action flight game; depending on how nice the coop mode is, it goes from day1 + joystick bundle to bargain bin)
Overlord (nice sense of humor, will wait and see how the final game ends up)
Bioshock (hype?)
DS
Dragon Quest IX (coop JRPG ^___^)
Front Mission DS (SRPG remake, the localization better be real)
Luminous Arc (another SRPG, can't have too many portable ones)
It's a Wonderful World (interesting-looking JRPG, gameplay/combat looks compelling)
Heroes of Mana (RPG-RTS, I'm still on the fence with this one)
Phoenix Wright 3 (no-brainer)
Phantom Hourglass (Zelda, and I'm interesting in seeing how the stylus controls turn out)
Megaman ZX Advent (2D action <3)
Ninja Gaiden DS (because Ninja Gaiden is so awesome)