I did not know about the loop mechanism. Thanks to all.
I use this trick to create a local yum repo for Fedora Core. I mount all the iso's to /mnt/FC4, then create a symlink farm under /opt/Fedora/FC4/RPMS to the RPM's on the ISO images. I run createrepo to create the metadata and then edit the config file in /etc/yum.repos.d to point at it.