As previously speculated, it looks as if Activision studio Treyarch will indeed be working on the next installment in the Call of Duty series. A job listing on the studio's website denotes the following:
[blockquote2]Treyarch (an Activision studio) is looking for talented and motivated Level Builders to join the team that developed Call of Duty 3, Call of Duty 2: Big Red One, and Call of Duty: United Offensive! We have positions open for both Multiplayer and Single Player builders. We are located in sunny Santa Monica, CA, close to the beach.[/blockquote2]Furthermore, the job listing goes on to add "if you're a fan of World War 2 shooters, then even better!" It's not much to go on, but certainly suggests that the series will lurch back into the era of World War II.
Level Builder - Call of Duty [Activision via Gamer.tm]