You might want to take a look at:
http://pfranc.com/usb/usb.shtml

This is a cool site started by a guy selling kits to make the proprietary connectors for various Garmin GPS receivers. I bought a couple of connectors for my Etrex for about 5 dollars, soldered them to junked mouse cables and saved 40 bucks.

He sells a USB to serial port adapter that looks like it might be the same one mentioned in a previous post, for less money.