Getting the Epson Perfection 3940 scanner setup on Linux requires jumping through just enough hoops that even if you have managed it before, it's easy to forget when it comes time to do it again.
I put this here in the hopes that it will make things easier for someone, (and it's entirely likely that someone will be me one day :).
adduser youruser scanner).
sudo aptitude install xsane).
rpm2cpio iscan-firmware-18.104.22.168-10.i586.rpm | cpio -i --make-directories).
sudo install -m 644 usr/share/iscan/esfw52.bin /lib/firmware/)