Well, today I needed it. I rolled up my sleeves and got ready for frustration, but it wasn't that bad. Contrary to what I thought, there is no driver that I needed to install for the USB to parallel adapter. In fact, it was already installed.
~$ ls /dev/u*
/dev/urandom /dev/usblp0 /dev/usbmon0 /dev/usbmon1 /dev/usbmon2
It is right there as /dev/usblp0. But the printing system in Gnome seems to know nothing about it. So how do I add my printer?
Assuming you are running Ubuntu 10.04 as I am: Select System > Administration > Printing, click the Add button, then select Other in the devices list. Now enter the magical device URI:
Add your printer using the built-in Linux driver and voila!