Apple indicates this is a OS update error to 10.5.7 (1) but have seen it elsewhere. When you try to make a change to your printing settings, add a printer or sometimes starting a print job, a dialog box pops up asking for a user name and password for a user in the lpadmin group.

To correct this, you must enter Terminal and type the following when logged in as administrator.

dseditgroup -o edit -p -a admin -t group _lpadmin