Quoting from http://www.ancientpond.com/ljshen.html:

The LJ has a number of legitimate messages built in that pop up in cases of low toner, paper out, paper jam, etc. There is one message that appears while there is no error condition: READY. This "Readymessage" indicates that everything is fine, and no intervention is necessary. Now, what a lot of people don't know is that this message can be programmed, by sending a simple escape string to the printer...

A quick recipe for the *nix users (Macs, you need the OS X Developer Tools) -- certainly not the simplest way, but worked for me. We print directly to networked printers, so the whole Windows print server name/domain stuff is N/A. This assumes you know the IP of the printer.