As long as the screen and the usability is the same I don't see it as a huge problem, just like a computer programs run on a variety of different speeds of hardware.
As far as games go the version for the older machines would run with lower poly counts, textures and whatnot. Other productivity apps might just run a little slower.
The speed has been the same for almost 2 years now, they need to up the speed a little or they will fall behind the competition, we don't want just another RAZR.
As long as the apps run on both I don't see a problem, and then they can up the speed again in 2 years which then all the slower iPods/iPhones would become semi-obsolete depending on if developers wanted to support them. And it's not like those iPods would not function anymore they would just get less new apps.