After a fresh installation of Fedora 15, I ran "Software Updates", added the "Fedora 15 - x86_64 - Updates" repository, and tried to run the update. It failed with a "transaction error" and the following details message: "could not add package update for fedora-release-rawhide-15-3(noarch)updates: fedora-release-rawhide-15-3.noarch"
This is actually a PackageKit bug, see bug 716752. See https://fedoraproject.org/wiki/Common_F15_bugs#.22could_not_add_package_update_for_fedora-release-rawhide-15-3.28noarch.29updates:_fedora-release-rawhide-15-3.noarch.22 for the procedure to work around this. *** This bug has been marked as a duplicate of bug 716752 ***