Valve's teasing has ultimately culminated in the Mann Vs Machine update, a players vs Robots mode that joins Red and Blu in holy matrimony.
The Robots will be slaughtered in a "Horde" style mode where five players can team up to beat down on waves and waves of robots that sound like the go-karts at Golf 'N Stuff off the 101 freeway in Ventura, CA. The robots are, of course, modeled after the iconic team mates you've come to know and love and murder and murder and murder, so this shouldn't be too difficult of a switch to make.
The mode also brings with it new achievements and the opportunity to upgrade your abilities and weapons in between rounds. Loot waits at the end for any survivors, probably wearable loot that makes your character look funny… like a pirate or a disco afro dude.
The update is scheduled to go live on August 15th with more details on the mode scheduled in the coming days.