At the low end AMD could give a solid CPU/GPU/chipset combination, maybe even a custom job which would be something Apple would like. You know, make it a bit more difficult to compare internals side by side.
For now, nvidia chipset with integrated GPU works just fine for the macbook and mini. But core2duo won't be enough after a while. And as Intel still can't cut it in the GPU front, and discrete graphics are undesireable from the size/heat/costs viewpoint, it's natural to look for alternatives.
If Intel were to give chipset licence to nVidia, Apple wouldn't be going this direction, so maybe it's just Apple adding pressure. But with HP and the likes already using AMD for similar reasons, it's unlikely to cause Intel to cave in.
Mid-range, iMacs and such, AMD would have good enough processors right now. And as for a complete package, nVidia really isn't pushing small, cheap, low-wattage GPU's nowadays is it. So if the future GPU's are from AMD, why not grab the whole CPU+GPU+chipset package?
High end, I doubt it. But maybe there's something in the not so public roadmaps we don't know about yet.