So, Google has 'fessed up how they've been building their database of cell IDs. Apple use such a database of cells to location-enable the iPhone v1 mapping applications; whilst the spangly new iPhone has onboard GPS to improve accuracy in this respect, I've found cell ID to be extremely useful when combined with a decent map.

And for the rest of us, there are stirling efforts like OpenCellID to help build an open database of cell ID to lat/long mappings... with a nice downloadable client to let those of us with GPS-enabled phones to contribute.