Hey, have you read the news today about Google's new product "Latitude"?
Many people have GPS units in their mobile phone and don't realize it. This is a new version of Google Maps which will allow you (at your option) to share your position with others. It's easy to go in stealth mode, if you wish.
In can be found at
Google Latitude
So far, it seems really cool. I've already shared it with Pete and have told my wife she can now keep tabs on me. :P
Harvey