Pretty slick, dcb--you caught me; I hadn't looked

. Although I don't know that there's a good Arduino-environment-to-bootloadHID bridge yet (although if not, I'm sure there will be-- I just haven't looked).
Now, this does still require building the external USB circuit for the AVRUSB usb port, right? And since you're using AVRUSB there are still the other problems mentioned (manually calling refresh() periodically, which you may still have to do with LUFA (I haven't checked). And there are possibly some other issues. So in actuality this doesn't change
my usage much (my shield will still be essentially the same) but it's a great alternative to know about and if I was building an end-device I can see how you could set up sketches for serial port monitoring and stuff. I'm academically curious to know if it flashes any faster (heh), but that's obviously not a big deal.
Cool development, though--thanks for being more insistent on bringing it to our attention!