http://www.youtube.com/watch?v=kVPjhmTThPo&NR=1
I found something called the NuVinci transmission. It works similarly to a ball diff. Instead of one ball, there are three and as the hub spins faster, the cintrifucal force adjust the gearing, and it gets taller...
Any chance something like this will ever be made for rc use? Looks really efficient...