What GPU functions or capabilities are behind AirPlay mirroring?
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.