Just realized that while Linux is up and coming with some USB support, FreeBSD has whats considered very stable implementations done. Shouldn't be too hard to get that ported to PicoBSD, (I think).
Considering the fact that PicoBSD can be up and running from a 1.4MB image, would leave quite a bit of free memory to play with.
Ohh, you just "gotta" have that gui, don't you :)
Seriously though, haven't played with *nix's or BSD much at all, so your mileage may vary.
Just thought it might stir up some discussion.