So far, Apple have given no indication that they will drop support for Core Duo or Core Solo machines. I'm not saying they won't, I'm saying there's no sign of that at this point and I don't think those machines are quite old enough for the chop just yet (opinion).
There's also no technical reason for Apple to drop support for any model based solely on video card/video chipset, as the OS doesn't really require much firepower in this regard, so I doubt that GMA machines will be unsupported either. They obviously won't perform as well, and future Apple *retail* software will probably drop support for such machines, but there's no technical reason for these chipsets to be unsupported in the OS until Apple moves to an entirely new graphics engine/goes 3D like Avatar.