Appreciate the good words about the software, Toby -- really glad you like it and put it to good use, especially in publishing highly useful GPX files for fellow bikepackers.
I do want to clarify that while TF can upload custom maps (raster images... topo, aerial, etc) to the eTrex 20/30 (and many others) it doesn't deal with the vector basemaps like Garmin's Topo USA set does. Luckily there are free options for most states available, but the Garmin set is kind of nice if you want to upload a huge area and be done with it.
Here's a good collection of the free stuff:
http://www.gpsfiledepot.com/maps/state/all