Looks like field trials are underway...
http://blog.apertureacademy.com/2012/01/canon-200-400mm-and-600mm-prototype.html
http://blog.apertureacademy.com/2012/01/canon-200-400mm-and-600mm-prototype.html
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According to Canon Rummors this guy claims he is using already release cameras and lens. However, button locations, etc don't match up. The rumor is an announcement Feb 7th I guess we will see.