tiny RC receiver for bristlebot?
I recently finished my library for the RFM70 radio module, check http://www.voti.nl/rfm70 I have tested it (ao.) with an arduino. That module is cheap, and has a digital interface. Negative points might be short range, and 3V3 operation.

But why do you rule out IR? You could use a existing TV remote, saves you half the effort.