[Michael Hill]

There are quite a few options. I don't think I've seen a team that's done it (but I don't know how you COULD see a team do this), but if you want to take into account slippage, you could attach a >= 2-axis accelerometer as well as an encoder. You will notice the behavior of slippage in an accelerometer (or the lack thereof behavior). You can use that information as a sanity check for your encoder. If you see your encoder moving and you know you should be accelerating but don't see it in your accelerometer, then you know you're slipping. As mentioned before, an unpowered 5th (and 6th) wheel would work just as well.

I think there are also options for the ultrasonic sensor.