Activision has been using a strategy of alternating studios in their Call of Duty franchise. Treyarch developed Call of Duty 2 and Call of Duty 3. Then Call of Duty 4: Modern Warfare was made by Infinity Ward. It was Treyarch's turn again with Call of Duty: World at War and then Infinity Ward is hard at work on Modern Warfare 2 which may be dropping the "Call of Duty" title in favor of becoming a "Modern Warfare" franchise.
If you followed all of that then you would not be surprised to hear that Treyarch was in line to make what would be Call of Duty 7 if you were going in order. In fact that appears to be the case according to Treyarch Senior Animator David Kim's LinkedIn profile. Before being updated today, Call of Duty 7 was listed twice on his page.