Interesting. Can you tell me which tools you use to work around Garmin? Mac compatible? Are their formats proprietary? Would be stoked for a solution.
The map format (.img) is proprietary, but has been reversed engineered, so, for example, there are tools for converting OpenStreetMap data into the format. You can download maps of many areas from cloudmade.com.
I usually download my logs onto a Linux machine, but it works just the same on the Mac (using the device as USB mass storage).
Dealing with Garmin devices is only tough for the new fitness devices...proprietary binary format :( I wrote a ruby module in C to stream parse it which I plan to opensource soonish.