Just thought I'd announce this here, in case it's of use:
About an hour ago, I checked in a PHP script which checks the database
for users who haven't made edits, and deletes them. It runs from the
command line (phpShell seems fine with it, too) and works relatively
quickly.
You'll be able to find it in both HEAD and in REL_1_5; look in the
maintenance directories. This will work with 1.5.x and 1.6, and is
impossible to misuse. The first user in the table won't be touched,
ever, as that's usually the site owner and default bureaucrat on most
wikis. I'm considering adding functionality to remove users who
haven't made an edit in X amount of time, too, which won't be too much
trouble to add, I shouldn't think.
Enjoy,
Rob Church