Update:

As we reported earlier from various rumors, Call of Duty 5 has now been confirmed by Activision to be set in a new theater.

Activision announced today fiscal year 2008 and fourth quarter results, confirming that the fifth installment in the Call of Duty series will arrive this fiscal year.

"We’ll launch on all four platforms we’ve participated on in fiscal 2008, we’ll launch on the PS2 and the Wii." said Activision Publishing president and CEO Mike Griffith.

Griffith also confirmed that Call of Duty 5 will be set in a "a new military theater"

My gut says that it’ll be the Pacific Theater. What do you hope for?

Earlier:

The next Call of Duty will arrive across six different systems before the end of Activision’s fiscal year, which ends March 31, 2009, and will feature a "new military theater," according to Activision Publishing CEO Mike Griffith. The undisclosed new setting will try to capitalize on the massive success of Call of Duty 4: Modern Warfare which, according to an Activision earnings call, has doubled the install base of the franchise ever since the company took the series out of World War II. Next-Gen.biz reports that the next installment of the best selling first person shooter will appear on the DS, PC, PlayStation 2, PlayStation 3, Wii and Xbox 360 platforms.

No word on whether Infinity Ward, the developer of Call of Duty 1, 2, and 4 will develop the game. Up until recently the next Call of Duty was speculated to be developed by Treyarch, the developer responsible for Call of Duty 3, and was rumored to once again take place during WW II. The rumor surfaced when Treyarch looked for a new company position:

"Working experience with First-Person Shooter or Third-Person Shooter genres (console or PC). And if you’re a fan of World War 2 shooters, then even better!"

Source