I think rotary encoders maybe my answer. I'm not entirely sure how I'd fit them to the chassis I have. The wheels are based on rugged Tamiya RC kits.
I was considering the RF beacons, it definitely pandered to the geek in me but I want an entirely autonomous bot which doesn't require any major setups to pull off navigation.
If people can build UAV's then this technology must be reasonably available to hobbyists. You'd have to normalise the results across multiple sensors but I'm positive you could accurately measure inertia to provide X,Y and an angle (Other degrees of freedom don't concern me at the moment)