I'd be interested to see how well the high power versions of those gearmotors from Pololu worked in a beetleweight, as they have far more power than an ant can actually put down. They're about 12W each peak, so I'd think 4 of them would happily power a beetle.

Those ones on ebay are unlikely to be the high power version though, probably more likely the 4W version, which wouldn't be much use in a beetle I'd have thought.