Comment 26 for bug 121082

Thank's very much, I had the same problem and added
# CanoScan FS4000US Film Scanner
ATTRS{idVendor}=="04a9", ATTRS{idProduct}=="3042", ENV{libsane_matched}="yes"

to /etc/udev/rules.d/45-libsane.rules, otherwise vuescan will not detect that filmscanner (except when used as root). For sane it's useless as that scanner is not supported by sane.
In my case (karmic) /etc/udev/rules.d/45-libsane.rules did not even exists, therefore the only thing to do was to add that line and to reboot. No scanner group was necessary but I'm member of plugdev.