This is a very recent forum on Makezine.com about using your Garmin GPS device to downlaod your coordinates on to your PC so you can use 3rd party maps / applications.
“I have a Garmin eTrex Legend. It has a PC Cable which looks to be nothing more than a DB9 serial cable. I believe the intended purpose of the cable is to upload the ~$99 Garmin proprietary map packs into the GPS. Is it possible to go the other way — to get my coordinates from the GPS into my PC? Has anybody done this? Is there a published protcol or, better yet, a library for talking to the GPS? The idea here is that if I can get my coordinates, I can use 3rd party maps on the PC.”