So far they've just said that there is a co-op invasion mode. We don't know what all is in the invasion mode though. One of the developers was in the hvs chat yesterday and mentioned that there was some stuff other than just a horde mode, but of course he didn't want to get into too much detail. He mentioned that we may see stuff along the lines of bosses, loot, and something else that I can't remember.
Everything I've learned about life can be summed up into three words: It goes on -Robert Frost Playing: Golden Sun TLA, Borderlands, MW2, Civ IV