When the iPhone came out I had a great idea to actually implement this... unfortunately there were too many road blocks, the main one being that the carriers don't tell you where the cell towers are. <a href="http://tlrobinson.net/blog/?p=24" rel="nofollow">http://tlrobinson.net/blog/?p=24</a><p>A company called Navizon ( <a href="http://www.navizon.com/" rel="nofollow">http://www.navizon.com/</a> ) has such a product, and they rely on users to wardrive for WiFi and cell towers.
Really nice! Seems that they have also made a Symbian S60 version of the app, used to be Java only (J2ME).<p>Finds my position in Finland with ~1 km accuracy, and it is actually quite fun to see where the tower you are connected to is. For me that changes when I change my position in my kitchen...