Patch Release.
The game's online co-op prior to the patch was in a word, "broken", but today hopefully spells the end for those woes. The patch is said to include a number of other fixes:
Saved data loss issues
Quick Match/Custom Match multiplayer games not working/displaying error messages
Xbox LIVE games dropping unexpectedly
Arena Master achievement will now unlock after any mixed arena matches.
Overall network performance improvements
A few crash bugs
NXE related issues
Max experience cap lifted
Boomerang exploit is now resolved
Alien Hominid can now drop the pitchfork!