* iMac (Mid 2011 or newer)
* Mac mini (Mid 2011 or newer)
* MacBook Air (Mid 2011 or newer)
* MacBook Pro (Early 2011 or newer)
I remember reading that AirPlay mirroring uses the video encoding capabilities of SandyBridge CPUs, but I can't find a source for that now.
Anecdotally, http://airparrot.com really taxes my 2010 MacBook's CPU.