I know a lot of instructions exist about how to setup Mustek 1248ub scanner in linux. All of them (from Ubuntu forum) refer to this webpage, recommend you download appropriate driver, save it into /usr/share/sane/gt68xx folder, add read permissions to all and be happy. But none of them told to uncomment gt68xx line in /etc/sane.d/dll.conf file! So, do it and sane-find-scanner will do it best for you!
Also there are such situations, when sane-find-scanner does not find scanner, but scanimage -L and xsane do well!