[FotoPlasma]

I've thought about this a bit, and on the plane back from Houston, this year, I thought that if you could monitor rpms at a given wheel, and also monitor the current going to the motors powering that wheel, you could compare the two values and experimentally determine whether your wheels are slipping. If you're drawing just a few Amps (some arbitrary low amount) while you're seeing high rpms, you might be able to safely assume that the given wheel is slipping.

Just something that might help.