According to the Los Angeles Times Activision has tasked a third unnamed development studio to work on the Call of Duty franchise apart from Infinity Ward and Treyarch. It is unclear whether this third studio will replace Treyarch working on the titles in between original creator Infinity Ward's effort or if they are moving to three year cycles on the projects. According to the same newspaper Activision are also looking into a subscription based Call of Duty MMO.
Rumours on the internet point to newly founded Sledgehammer Games as the likely third Call of Duty studio. Sledgehammer Games is headed up by the former Visceral Games duo Glen Schofield and Michael Condrey who worked on Dead Space among other things.