c't features a very informative article about personal navigation in its latest issue (unfortunately the article isn't available online yet).
It presents four more or less practicable ways of a personal navigation system. I had put some research into this area in back 2001 with the emergence of GPS systems for PDA's. Indeed the idea to have a personal navigation in your mobile all the time is intriguing. You will be able to navigate in any point on earth as you would be local (besides not knowing the real shortcuts).
GPS becomes available on mobiles via an inbuilt receiver in Benefon's and Garmin's product. However these are non mainstream products. The handling is less appealing to non-outdoor fans.

Wayfinder and Jentro approach the issue with a simple and small extra GPS receiver connected via Bluetooth. All navigation data is stored in a central system. Both applications connect via GPRS or SMS with the hosted data center and receive routing information. You will be guided then via your headset or via display information. There is no need for any preparation of a trip or downloading of maps.

Wayfinder offers a very clever solution and is ahead of any other product in Europe. If a simple GPS receiver will be integrated in most mobiles in some years, revenue might take off with the company's approach.

I'm quite sure this can be predicted as a new killer application. Just consider calling the yellow pages directory services and instead of a street address the route to the nearest service point will be shown in your mobile. You will never miss your favorite hotel chain in an unknown city. You will not miss your favorite chain of restaurants. This is a great opportunity for companies to ensure full brand loyalty.
Just press the red button and the route to the nearest McDonalds will be shown. Enjoy!