Why is the Print button in the Print dialog greyed out on RHEL?

Issue

• This seems to have started once we updated our Cups server from RHEL4.6 to 5.5.  Printing generally works ('lpr /etc/group' prints fine, as do PS files), and I can print-to-file from Firefox, but Linux desktops can no longer print from Firefox normally.
• Seems no matter what printer I choose, the Print button is greyed out.  General command-line utilities seem to work- lpq, lpr for example.