I got an AppleTV for our office and brought over my old Sharp TV which is able to display 720p well enough to be used as a presentation display via AirPlay. When I tried to use AirPlay to display the desktop of a MacMini server I discovered this bug.
The workaround at the moment is to use the AirParrot software which has no problems displaying the desktop. I filed it as rdar://12167290 and mirrored it on OpenRadar.
Update Nov 7th: Apple closed the bug report as a duplicate of rdar://11782381.