I've wondered about this myself. After all, I doubt there's something different about a black man's physique that makes him more suited for running back than a white person. Personally, I think you can take it back to the high school level. Take two players, one white and one black, with the exact same skill sets. The black player is turned into a running back. The white player is turned into a safety, or told to pack on a few pounds and become a fullback or tight end. I don't know where this comes from, but it's the best explanation I can come up with.
It seems as if the only way a white player can become a starting RB is to hang back at FB and wait for guys to get injured. Sam McGuffie, a freshman RB from Michigan, might be the next legitimate white speed rusher.