Thanks for the attempted assist, kdarling. Unfortunately, this is happening to me as I drive all throughout the South Bay area of Greater Los Angeles on the 3G network.
Checking the Apple iPhone forum, apparently other people are having your problem in Torrance and Redondo Beach while on 3G.
Is your GPS turned off? Are you only using cell id locating? If so...
Then another possibility is that ATT accidentally or on purpose used a Boulder cell tower id for Torrance 3G. Or Google's database of cell ids got goofed up.
(Cell id changes are not unknown. In some countries, carriers would swap cell ids every day, mostly to prevent people from using cell id databases to get out of paying the carrier for locating services.)
You need to open a trouble ticket with ATT, and also go to Google Maps support and send them a note. Send feedback to Apple in their site as well, just to get more attention. Again, assuming you're using cell id locating and not GPS.