Jim Hughes has pointed me, quite separately at two resources: the latitude and longitude for cell mast transmitters and some Python code for viewing Google maps via your Symbian phone
Given these two pieces of information you should be able to create a phone-based Google maps browser which centres on your closest transmitter.
You should also be able to push your location (or rather the location of the nearest transmitter) back up to a server as well when you‘re out on the road. I’m not sure what the accuracy of this would be, but in lieu of good, cheap GPS devices, it’s not bad at all.
The Google maps thing in particular would be really cool if it could show you an annotated map so, for instance, I now work in Bath and don’t really know where the shops in town are (or pubs even) so I could use an annotated version to navigate through those. Turn it on, and it should automatically come in close enough to where I am for me to be able to recognise exactly where I am and from there I can use it to find the shops I’m looking for.