According to the Official Xbox Magazine, Call of Duty 5 will be released as Call of Duty: World at War, and will revert back to the WWII theatre upon its release in early 2009.
Treyarch Invention, the studio which developed Call of Duty 2: Big Red One and Call of Duty 3 are heading up the design, and the game will use the Call of Duty 4 engine.
Call of Duty: World at War will take place in the Pacific theatre (and partially in Russia's advance into Berlin). "This is a scarier Call of Duty than we've ever seen," Treyarch head Mark Lamir says in the magazine article.
For the first time in Call of Duty's campaign 4-player co-op is supported over Xbox Live - and it supports meta game perks as in CoD4's online mode.
Call of Duty: World at War will be available for PC, Xbox 360, PS3 and DS, as well as for Wii and PS2.