Canon USA has issued an update on the availability of its 500mm f/4L IS II and 600mm f/4L IS II lenses, which were previously announced in February 2011. Initially, the lenses were set to be available in May and June 2011; however, that was pushed back until December 2011 and ultimately listed as TBD for the availability of both models.
Now, Canon says that both lenses will be available in May 2012. Currently, B&H lists the availability for each lens as April 25, 2012. Check the following links.