Here's a lengthy explanation from Pat:
Changed udev rules to use group lp instead of scanner. It turns out that this is likely what's been interfering with printer detection since we switched CUPS to use libusb instead of the usblp kernel module. SANE would come along and switch the group on multifunction printer/scanners to "scanner", and then CUPS would no longer be able to use them. The kernel module wasn't affected by this since it didn't use the raw USB device node and didn't care who owned it. Anyway, making this switch gets things pretty close to how they were before. The good news is that printers should be detected and configurable without having to look at lsusb output or writing custom udev rules, however, users will need to be in group lp in order to use the scanner. We'll do this automatically for console logins.