which kernel do you use? Do you have scanner drivers in kernel? There is a quick way to change the USB permissions on 2.6 kernel. In your /etc/fstab you should have a line like
usbfs /proc/bus/usb usbfs defaults,devmode=0666 0 0
the option devmode gives everyone the read-write access to all usb devices. But beware, there was a bug in kernels < 2.6.8 so it did not work. There is a better but a bit more complicated way of doing it, using hotplug.
Things may be different, though, if you have corresponding kernel drivers installed.