While looking through the press kits for all the stuff Microsoft showed off today, I came across a fact sheet for Geometry Wars: Retro Evolved 2 and it will indeed have 4-player online co-op.
Update: Yes the bad news is it won't be online co-op.
The game will feature 5 new game modes, including a Co-Pilot mode where one player steers and one player shoots. Geometry Wars 2 is set to drop exclusively on Xbox Live Arcade on August 6th.
We've also posted some new screenshots after the jump.
