Thanks for the replies, and the link to the Fedora yum docs. :)
Unlike up2date and yum, system-config-packages installs software packages from your Fedora Core installation discs only, and does not use repositories.
Seems like it's time to put up2date/RHN/system-config-packages out to pasture. Even RHEL could probably just use yum instead, but be set up to access RHEL customer-only Redhat repos.
I didn't install yumex. I've tried it briefly in the past, but found that the yum command was just so simple that it didn't really require a gui front-end.