You should upgrade regardless, especially if you're running Lion today.
Requires a second-generation Apple TV or later. Supports the following Mac models:
iMac (Mid 2011 or newer)
Mac mini (Mid 2011 or newer)
MacBook Air (Mid 2011 or newer)
MacBook Pro (Early 2011 or newer)
That's a let-down. My iMac is a 2009 I believe, and AirPlay Mirroring was the main feature that had me contemplating ML. Can't help but wonder though, why the older computers are incapable of handling AirPlay. My iPad can do it, I figured it'd be easy for my iMac.
She who laughs last -- probably made a back-up.
“For the modern [Intel and AMD] chipsets Apple has written drivers to access the on-GPU H264 encoding,” says Stanfill. “Since the image of whatever is on screen is already in your graphics’ frame buffer, all Apple needs to do is the scaling (the most expensive part of AirPlay Mirroring, which is why you have ‘Best for AirPlay’ resolutions that are 1:1), the color conversion (H.264 video needs YUV color space vs. RGB), and the actual encoding itself right on the GPU. Then, Apple only has to transmit the relatively small encoded video stream to the Apple TV.”