Kelly Miller wrote:
Red Hat uses Fedora (good as it may be) purely as a test-bed, where they can try out new technologies that could prove to be too unstable for RHEL without any risk. Fixes from RHEL don’t go upstream to Fedora because the codecase it too different.
http://interviews.slashdot.org/article.pl?sid=06/08/17/177220 Question #8, "Goals", answered by Max Spevack.
