From what I read its not 100% compatible, but I think its a sliding scale...so it'll work just with some bits greyed out?
Basically some shaders not in that hardware (like its Direct X 8.1 not 9)...i.e. its probably not the end of the world..
*edit* discussion http://forums.macrumors.com/archive/index.php/t-100192 people whinging but some talk of scaling back/doing in software not hardware...
Apple's core image page http://www.apple.com/macosx/tiger/coreimage.html