The brochure at http://db.glug-bom.org/wiki/index.php/Linux_Brochure says: "Fedora Core is an RPM based Linux distribution, developed by the community-supported Fedora Project and sponsored by Red Hat. Its goal is to be a complete, general-purpose operating system that contains only free and open source software. It also considered to be the testing ground for future versions of Redhat Enterprise Linux."
According to a Fedora Project member in December's issue of LFY, Fedora Core is the upstream of RHEL and many of the other Fedora 'based' distributions like Scientific Linux, etc.. The person was categorical about Fedora Core not being a 'beta' or 'test-bed' for RHEL.
Cheers, Debarshi