This had bothered me as well and a Google search did not reveal an answer. (Turns out, simply cntl-clicking on this icon and reading would have.) So here’s to you Mr. TUAW Tip Guy.
I just ran across this simple, but handy tip over at Mac OS X Hints. One of the little annoyances of Leopard that didn’t make our recent post is the way Leopard handles printers. When you print something it starts the little application that controls the print queue for that device but (unlike Tiger) the application remains on your Dock after the print job is finished.
If this annoys you (like it does me), it turns out that there’s a simple fix. Just right click on the Dock icon and select “Auto Quit.” Now the printer queue application will quit itself (and so disappear from your Dock) once it’s finished processing your print job.
I’m not entirely sure why this behavior is not the default (as it was before), but it’s a nice little discovery that removes yet another little annoyance.