Satellite navigation for bikes?

Yesterdays ride ended up about 120km. My legs are very very tired, but it was a good ride. Good preparation for our big ride in a couple of weeks too. Gonna be a few days around that distance!

Part of the reason for the extra length yesterday was Lonely Planet's shitty directions and maps. It got me thinking: why hasn't someone come up with a satellite navigation system for this kinda thing? I guess cost is still too high, and probably not enough demand compared to cars.

For city cycling, though, it should be a relatively easy matter to overlay the government-provided cycle maps onto an existing map of a city. The navigation engine would be programmed to prefer the cycle routes, avoid motorways and A roads, and prefer shorter distances. I guess it could get difficult in hilly cities, a bit more complex as going downhill is a very different thing to uphill.

Perhaps something for the hobbyists to start?