How odd, I was just reading a direct mail peice this morning from DeLorme. The offer their Earthmate GPS that can plug into a PDA. It works with both their Map USA (roadmaps) and Topo USA (topographical maps) products. You'll also need proper cables to match your PDA brand as well as a secondary desktop program to migrate maps and info back and forth between the PDA and desktop.

One cool feature is the ability to capture GPS readings along your trek, and then upload them onto a map on your desktop. This allows you to save your actual path and calculate distance traveled and changes in elevation, as well as any anotation notes you might want to add for future reference.