Mobile GIS is becoming more interesting and easy to get into. Yesterday Navicore released their navigation software for the Nokia 770.
Maemo Mapper is a free and open-source mapping application, designed from the ground-up for the Nokia 770. However, a large caveat is that it uses GoogleMaps in what is probably a violation of the terms of service (realtime navigation and downloading), not to mention annoying in that you have to "pre-drive" your route to cache the appropriate GoogleMap tiles.
I've gotten slightly involved in the new GeoClue project - an effort to provide an easy "location service" backend for devices. The location on the device may be served up by GPS, WiFi, GeoIP, Mobile Cell/GSM, or even just the user clicking on a map or entering an address. Then, an application can subscribe to the location service and get updated with the current location of the user/device and use it as appropriate.
At FOSS4G I attended a BOF (Birds of a Feather - people interested in the same stuff) on Mobile GIS. The software and technology all exist, it just needs some coordinated efforts to define the use cases, interfaces, and approaches. However, one solution won't fit everyone. There are users who want to do "real GIS" in the field and there are users who want to do "neogeography" to say, find the nearest coffee shop on their mobile.