Some interesting developments.
Two people both with the 512MB version of picsel, the cards have different ID (if indeed /proc/driver/mmcsd/00/raw is the card ID (nothing else in the /proc/driver/mmcsd/00 directory looks especially interesting, and this file is read by pic-browser not long before it pops up the insert picsel card message).. the IDs are 60% the same though, the latter half varies.
You can't change the value in /proc/driver/mmcsd/00/raw, and since the Sharp's mmcsd driver is closed source I don't think there's a way to hack the module to allow you to spoof the value (or write to it even if it doesn't actually touch the card).
The binaries, as installed, both have the same md5sum, which I didn't expect.
--edit, fixed typos. seem to be making a lot recently