I dug up an old thread last night on the Adafruit forums where I had talked with some others about this. As it turns out, someone seems to have been able to figure out the checksum algorithm and posted the details a few months ago:
I haven't quite had a chance to go through it in detail yet, but it looks right to me. Once I get a chance, I'll update this page with the specifics.
Thanks to Sebastian (Nibbler) over at the Adafruit forums!