Looks cool. It would be especially useful if you cached and stored the location of known cellids so that it could be used without switching on a network device. Along these lines, being able to populate the database from online would be a good feature.

Regarding sources of information, have you looked at this source?: http://openbmap.org/

It also contains WiFI AP location data, which would be another nice thing to integrate and give access to in order to narrow down the device location without needing to start up the GPS (not sure about the power-location trade offs here, but certainly if you're in a building it might be worthwhile, and equally if you are connected to a WiFi AP anyway no need to start GPS).