What percentage of the market carries Tom Tom devices? It sounds like a tiny, tiny percentage. What would I be doing there other than proving the market for someone with the stomach to make the real money on the iTunes store?
Basically get noticed with the Tom-Tom(more likely because of pre-established intent) and then come up with an app for other devices. Or like I suggest a browser based app( I don't know if this is currently possible, but am sure it will happen soon)