I don't see it either. In two years, apple will begin exclusively selling intel boxes. Until then, everything coming out will be gauranteed to work and be heavily tested for the PPC. At that point, some 90% of the people running osx will still be running PPC. There will be a vocal minority who rush out and buy macintels. There will also be a vocal minority who buy the last powermac, as it'll be the Best Macintosh Ever Made. The rest will use their computer until they need/want a new one.

In three years, things will stop being optimized for PPC, and things might be a little slower. However, if you're worried about speed at that point, you're already going to own a new computer.

Matthew