Hello,
since ive seen the twin motor gearbox over at sparkfun i wanted to make something with it.
and whats nicer as taping a battery and a arduino to its back?? 
more information including source: www.designer2k2.at
It is moving quite nice, especially for the total price when you look at the gearbox + tires + caster for about 14? 
im planing on putting some more sensors on it, so that someday it will drive around without hitting something :
designer2k2