Hey There guys
I'm running Mountain Lion on iMac late 2009(i5 quad core 2,66ghz processor) but i can't see AirPlay Mirroring in the Monitor panel. It was the top model of Late 2009 mine iMac so it should have the right requirements for ML
AirPlay mirroring is only going to work with 2nd gen Core processors according to Macworld:
" (Apple says that only Macs with second-generation Intel Core processors can use this feature.)"
http://www.macworld.com/article/165407/2012/02/hands_on_with_apples_new_os_x_mountain_lion.html