My friend Allen has been tinkering with the GPS receiver on his Nextel cell phone. These phones have a Java Virtual Machine, and you can write your own software to do stuff like report the location of the phone.
So Allen gave me a phone to carry with me. You can get a map of where the phone was last on right here.
I did this on the 2003 coast to coast ride and it worked remarkably well. I had coverage almost everywhere. The exceptions were parts of the Blue Ridge Parkway, Mississippi, and west Texas.
Allen gave me the cell phone again, and I am using it right now as I type this on the train! It can't get a fix, though, because I'm in rural Alabama.