I found this and of course, having the sense of humor of your average (or maybe below-average) teenager, I could not resist. I even considered rigging up a script to use nmap to find all of the jetdirect servers on our network and change them all at once.
I started with "OUT OF CHEESE" three days ago, but nobody noticed until today. Of course, you had to be over 6 feet tall to see the display, so that probably had something to do with it.

Some other ideas:
You still work here?
Ding, ding your to go order is ready
Luke, I am your father
What is that on the ceiling?
Uh, a little to the left, yeah, that's the spot
Can you scratch my itch, it's on the bottom paper tray
Feed me Seymour
Give it to me big boy ...
Faster, faster, faster ...
I love it when you touch me like that
I need a beer
Paper jam ... just kidding
Error 458932 please dial 911 immediately
When this printing job is finished you're fired
I considered the following:
ID10T ERROR
PC LOAD LETTER
HEY THERE SEXY
But considered that all but the second might get me in a spot of trouble.