I have the same problem on Lucid:
when I install cups, I can print, add printers etc..
However, after reboot nothing works, even the "add printer" dialog in system settings is broken (impossible to add printer).
The only thing I can do is to REINSTALL CUPS and other cups-related packages via synaptic.
I hope it will be fixed soon, because now non-admin users can't use printer at all.