The Buzz: Call of Duty 5 to be the last non-Infinity Ward developed entry in the series.

Next-Gen reports that an ex-Activision Underground employee (the studio which is putting the finishing touches on Quake Wars for PS3) has revealed that a new contract is in place which will see all future Call of Duty titles developed by Infinity Ward.
Infinity Ward created the first Call of Duty title, along with CoD2 and the phenomenally successful Call of Duty 4: Modern Warfare. In order to ensure the series saw an annual release Activision turned to internal studio Treyarch who could develop Call of Duty titles on a staggered development schedule to Infinity Ward - this saw CoD2 (Infinity Ward) followed a year later by CoD3 (Treyarch), followed by CoD4 (Infinity Ward) and should be followed this year by CoD5 (Treyarch). It's unknown if Activision will maintain an annual CoD release schedule.
On top of this Activision Underground is said to have been working on a Call of Duty spin-off which dropped the first-person shooter gameplay for third-person action adventure. The new title had been pegged as the start of a yearly series to supplement the FPS series. However, with a new contract in place with Infinity Ward the Activision Underground project was effectively canned.
There's been no official confirmation of the change in Call of Duty development duties from Activision or Infinity Ward.