Some more experimentation: DMESG is my friend...

If I unplug uba1 after I've opened it in konqueror (and closed konqueror), then plug it back in, its going and coming are seen, and it is renamed ubc1 (ubb1 still being present).

So the problem seems to be that the resources aren't being released when a USB device is removed.

I tried to "unmount" before unplugging, both via Konqueror and a root terminal; the answer both times was "device is busy".