A bit of futzing around with Image::Magick later, and we have threadpaper, a little Perl program which takes a Threadless URL and gives you wallpaper.
Usage: ./threadpaper [options] [threadless-url] --width WIDTH width of wallpaper (default: 1600) --height HEIGHT height of wallpaper (default: 1200) --center center image instead of lower corner --logo include color-matched Threadless logo --scale PERCENT scale image by PERCENT --file FILENAME output filename (default: /tmp/wallpaper.png)
The --logo option is admittedly a bit silly, but when I realized I could guess which color to use for the logo based on the predominant colors in the image I wanted to give it a try. You can grab the logo file it uses here though.
I've got some examples up there too.
For extra fun, just call threadpaper with a threadless URL with a random product number between one and 376 when you login and use the result as your wallpaper for that session. Endless surprises!