So I've got good news and I've got bad news. The good news is the upcoming sequel to Call of Duty 4, aptly titled Call of Duty 5: World at War will feature co-op play. This is a first and long over due feature for the series. The latest version of the game will be on the Xbox 360, PS3 and even the Wii and will feature vehicles and squad based elements in multiplayer.
Thus ends the good news.
The bad news is the game is going back to World War 2, the Pacific Theater to be exact. The game is also going to be developed by Trey Arch (Call of Duty 3) not Infinity Ward (Call of Duty 2 and 4). Call of Duty 3 is considered the weakest of the series so far, and the WWII genre is completely tapped out for First Person Shooters, RTS's, Flight Sims, Arcade Games, and just about every other genre you can think of.
Thus ends the bad news.
Thanks to rafoca for pointing out the Strategy Informer article.