A source within the industry who has previously leaked other information when I was writing for Xboxic (leaks include confirming Black Op's March 2010, Naming Kinect before E3 May 2010, First Gears 3 Images April 2010, amongst others) has informed me that an upcoming Call of Duty title from Infinity Ward will not be created on an all new game engine, and will not be released this year under the title of Modern Warfare 3!
It has been alleged that Infinity Ward are creating a game that will focus on the character Ghost who first appeared in Modern Warfare 2, this game will centre around the events leading up to the war in Call of Duty 4: Modern Warfare as a prequel, and then further reveal how Ghost first became a member of Task Force 141 based on the comic book stories written by David Lapham. In various parts of the game that will focus on the life of Ghost - players will revisit locations from both Call of Duty 4 and Modern Warfare 2, but this time from a different viewpoint.
Although entirely a rumour - the game is meant to be in development on the Modern Warfare 2 engine. Robert Bowling (Infinity Ward's Community Manager) was contacted, but has neither confirmed or denied the rumours.
When an official announcement from both Activision and Infinity Ward is made regarding the Call of Duty title currently in development - we'll be sure to post it.