A new website has been discovered that includes a countdown which is believed to denote when Call of Duty: Modern Warfare 3 will be revealed to the world - handily, it coincides with GDC 2011.
American media received dog tags containing the URL (findmakarov.com), the message "end the war" and the name Sheperd, one of the antagonists from Modern Warfare 2.
Currently there's no real indication as to who will be developing the title. It could be the re-formed Infinity Ward, Sledgehammer Games, Raven or even a combination of the three.
Source: That Videogame Blog