Google's "Maps for Mobile" makes it so you can automatically send your phone's location to their maps application without GPS!
After this light-weight maps application is downloaded and installed on your phone, you just type the "0" key to activate the new, "My Location" Beta service.
It doesn't work on all phones, but if yours is one that does, It will then get your phones location data from the cell tower serving it, and it will plot your location on the map (within 1000 meters or better). You can then search for a destination, get directions, and more.
The MIT Technology Review has an excellent article about the underlying technology, and other developments in this kind of non-GPS related mobile tracking. Interestingly, Google does use some GPS data on the back-end of this "My Location" application:
"Google also uses data from cell phones in the area that do have GPS to help estimate the locations of the devices without it. In this way, Google adds geographic information to the cell-phone tower's identifiers that the company stores in a database."
So instead of "triangulating" your location by pinpointing you relative to two other towers, Google is just using the single tower that is serving your phone, and then using other people's Maps enabled GPS devices to help fill in the gaps! It sounds like quite an amazing mashup & cross reference of geo-location data. Integrating all of it with the maps application making it work with relative accuracy is a remarkable feat.
Not surprisingly for Google, they expect the service to improve and more "intelligent" with time.
"As the database grows, says Lee, the service will become more accurate. It will never be as accurate as GPS, but he expects that it could eventually find a person within a couple hundred meters"
Here's a wild thought: Since Google is the Authority on "Search" (and privacy issues notwithstanding) wouldn't it be something if you could search for another person by dialing their cell phone number and then find their current location on the map ?!?!