After upgrading to 10.04 (maybe 10.10) printing to my HP Laserjet 6L became intermittent. On very rare occations it worked. It is connected via a Belkin USB to parallel lead. The computer is currently running 11.10.

After 18 months of trying this and that, the following fixed it for me:
lsusb
displays: Bus 002 Device 004: ID 050d:0002 Belkin Components
sudo chmod a+w /dev/bus/usb/002/004
and it is fixed! at least right now it is.

Apparently if I make "lp" the group, and add write access to the group:
there is no need to chmod again.

...someone elses example:
~$ ls -l /dev/bus/usb/004
total 0
crw-rw-r-- 1 root root 189, 640 2010-10-11 23:09 001
crw-rw-r-- 1 root root 189, 641 2010-10-11 23:09 002
crw-rw-r--+ 1 root lp 189, 642 2010-10-12 00:14 003