Comment by julien_p

http://www.apple.com/osx/specs/ lists the following macs as supporing AirPlay

  * 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.