No packages are available to be approved. This may be because a synchronization has never been performed or because your Software Update Services server is not functioning correctly. For more information, click About Software Update Services, and see the troubleshooting section.

OK, I run through the troubleshooting section, which points me to the Monitor Server section of the SUSAdmin page. I click on refresh to update the memory cache, which is the suggested solution. No joy.

The only thing I noticed concerning SUS was in the Event Viewer a warning about not being able to process a line in the 'settings.txt' file. After some process of elimination, it looks like the warning is unrelated.

Any suggestions for the next step in troubleshooting this?

From the microsoft.public.softwareupdatesvcs newsgroup
If you're running SUS RTM (non-SP1) then this might be a known issue. If you go to the SUS Admin and then to the 'Set options' page, change something (anything), click 'Apply', then change it back and click 'Apply' again, the problem should go away.

This problem has been fixed in SP1.