Currently I am working on a mobile device GPS navigation project (nearly finished), used by cars. The project is programmed in C# but can be running on PPC/Palm/PC, and can be extended to Symbian/S60/S80/UIQ and so on, with little changes.
So, I can do this project and succeed with strong confidence, let's talk more about your requirements. e.g.:mobile device OS, internet access?
Demo is available if necessary.