Capcom announced today that Left 4 Dead 2's Coach, Nick, Ellis, and Rochelle will be playable in Resident Evil 6's PC-exclusive Mercenaries No Mercy multiplayer mode, and in the other direction, three of Resident Evil 6's zombies will shuffle into the PC version of Left 4 Dead 2. The crossover, which is being called "the Resident Evil 6 x Left 4 Dead 2 project" (Capcom's idea, we presume) will be free for both games later this spring.
Not to spoil anything, but October 2nd is the release date revealed at the end of the new Resident Evil 6 trailer. More importantly, in that three minutes of undead sizzle you’ll find plenty of old friends from across the RE series (Ada Wong!), an introduction to Albert Wekser’s kid, a Far East location, and the horrific effects of the all-new C-virus.