The problem: with hplip installed and udev used (pretty standard setup), the /dev/usb/lp0 device is mode 660, owned by root:plugdev. This plugdev is troublesome, because it should be lp to allow printing by cups. This is what hplip's 70-hpmud.rules file was intended for, but it's ineffective. Solution: will attach a proposed patch which fixes the problem on my system.
Created attachment 176642 [details, diff] 70-hpmud.rules.patch This is a patch against /etc/udev/rules.d/70-hpmud.rules, which is part of net-print/hplip-2.8.6b, the current stable version for amd64.
Paweł: This WORKSFORME. Is it still a problem for you? i have net-print/hplip-2.8.6b and bangert@marsupilami ~ $ ls -l /dev/usb/lp0 crw-rw---- 1 root lp 180, 0 2010-01-15 14:56 /dev/usb/lp0
Let's close it for now and I can reopen in case I detect it again. It's on a machine I don't have remote access to, and rarely access physically.