Speaking at a Call of Duty: World at War event, developer Treyarch told Videogamer.com that there aren't going to be any numbers attached to Call of Duty sequels from here on in.
"We're getting rid of the number in Call of Duty for a very specific reason," said senior producer Noah Heller. "It's because we want you to know that when you're playing Call of Duty: World at War you're playing the best shooter, the best WW2 game ever. Likewise when you're playing Modern Warfare, likewise when you play any game that will be called Call of Duty."
There's a massive interview with Heller and creative lead Rich Farrelly right here.