My guess would be that it will depend on the Mac hardware. The Macs now should work fine, but if you buy a new Mac that has the rumored high pixel density displays... That one could have a higher screen resolution than ATV can support. Thus, you'd have to have the new one that can do 1080p.
Kinda like iPads. The ATV2 can AirPlay iPad2. Though, for iPad3, you have to have ATV3.
This is all pure speculation, but it's my guess anyways.