Back in the day, we had an app on Windows Mobile called CityID (Flashback: 2006!). It recently reared its dormant head on the Verizon Imagio (see full review). The app was basically an area code database that compared that was compared to an incoming phone call, resulting in a City, State caller ID.
Very useful stuff.
Turns out that many HTC devices, likewise, have a database that stores states, but not necessarily anything else. Saumaun, who is quite well known in various Windows Mobile communities, managed to whip up a detailed registry change to give us back that function. He basically included all the major metropolitan areas and referenced them via area codes, giving a pretty decent shot at city accuracy.
The best part of his "hack" is that there is no running programs or increased resources--it simply uses the device's registry to enhance caller ID. ROM chefs are encouraged to incorporate it into their builds and end-users can just download and install the simple and tiny .cab file, found in this thread.