The product page for AirPlay states that advanced hardware acceleration is used to make mirroring to an Apple TV possible. The footnote shows that only Mid 2011 Macs and newer can use this function. The graphics capabilities of many recent Macs are very capable, so has anyone figured out what exactly is being done or required of the hardware to do this sort of network mirroring? (Might the GPU be handling the encryption for the OS or some other very new feature of a graphics system?)
There's some misinformation going on at the moment on what exactly is needed for Macs to support AirPlay Mirroring. The cultofmac article correctly states direct access to the GPU for H264 encoding but this is supported on Macs older than the one's listed to support AirPlay Mirroring. So this isn't the main requirement.
The really important feature to make AirPlay Mirroring work is Intel's QuickSync baked into Sandy Bridge CPUs. And only the newer devices listed by Apple have that. So the main requirement for AirPlay Mirroring is a specific CPU feature, not GPU.
From what I understood, it requires H264 encoding capability at hardware level. See http://www.cultofmac.com/178460/the-real-reason-why-macs-before-2011-cant-use-airplay-mirroring-in-mountain-lion-feature/ for details