no printing with gnome print dialog

[Joe Smith]

If I try to print from Firefox (i.e. File > Print), or any other 
application that uses the new Gnome printer dialog (gedit, gimp), the 
"Print" button is disabled. If I select "Print to file", the "Print" 
button is enabled; switching back to an actual printer queue, it grays 
out again.

Other apps that don't use the new dialog print just fine; CUPS is not 
showing any errors.

Apps using (apparently) an older Gnome printing dialog, e.g. 
Applications > Accessories > Dictionary are able to print.

Is there something I can try to figure out why the print button isn't 
activated?

<Joe