The wordpress-mu package is at version 2.9.1.1, which contains known bugs, including a serious SQL injection bug (http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=605603). I'm not sure if there is a released wordpress-mu where this bug is fixed. It's probably worth starting talks with the wordpress maintainer about updating that to the latest version and dropping wordpress-mu, now that wordpress and wordpress-mu have merged into one project.
WordPress MU requires a bit of manual intervention to update from 2.x to 3.0; see http://developersmind.com/2010/06/17/upgrading-wordpress-mu-2-9-2-to-wordpress-3-0/ for one tutorial. It might be easier and less disruptive to backport security fixes for now, and target wordpress 3.0 for F15.
wordpress-mu-2.9.2-2.fc13 has been submitted as an update for Fedora 13. https://admin.fedoraproject.org/updates/wordpress-mu-2.9.2-2.fc13
wordpress-mu-2.9.2-2.fc14 has been submitted as an update for Fedora 14. https://admin.fedoraproject.org/updates/wordpress-mu-2.9.2-2.fc14
wordpress-mu-2.9.2-2.el5 has been submitted as an update for Fedora EPEL 5. https://admin.fedoraproject.org/updates/wordpress-mu-2.9.2-2.el5
wordpress-mu-2.9.2-2.fc14 has been pushed to the Fedora 14 testing repository. If problems still persist, please make note of it in this bug report. If you want to test the update, you can install it with su -c 'yum --enablerepo=updates-testing update wordpress-mu'. You can provide feedback for this update here: https://admin.fedoraproject.org/updates/wordpress-mu-2.9.2-2.fc14
wordpress-mu-2.9.2-2.fc13 has been pushed to the Fedora 13 stable repository. If problems still persist, please make note of it in this bug report.
wordpress-mu-2.9.2-2.fc14 has been pushed to the Fedora 14 stable repository. If problems still persist, please make note of it in this bug report.
wordpress-mu-2.9.2-3.el5 has been pushed to the Fedora EPEL 5 stable repository. If problems still persist, please make note of it in this bug report.