Addendum: I've found out that if you edit /usr/sbin/rebuildfstab using KWrite (instead of e.g.vim) it loses its executable status.

You can test for this from the command line by
Code:
ls -l /usr/sbin/rebuildfstab
and restore executability by
Code:
chmod 0755 /usr/sbin/rebuildfstab
(or by right-click on the file in Konqueror, select Properties, Permissions, check "is executable" box).