Fedora 18 changes the way how to work with services in spec files. It introduces new macros - %systemd_post, %systemd_preun and %systemd_postun; which replace scriptlets from Fedora 17 and older (see https://fedoraproject.org/wiki/Packaging:ScriptletSnippets#Systemd, https://bugzilla.redhat.com/show_bug.cgi?id=850016).
mdadm-3.2.5-8.fc18 has been submitted as an update for Fedora 18. https://admin.fedoraproject.org/updates/mdadm-3.2.5-8.fc18
Package mdadm-3.2.5-8.fc18: * should fix your issue, * was pushed to the Fedora 18 testing repository, * should be available at your local mirror within two days. Update it with: # su -c 'yum update --enablerepo=updates-testing mdadm-3.2.5-8.fc18' as soon as you are able to. Please go to the following url: https://admin.fedoraproject.org/updates/FEDORA-2012-13725/mdadm-3.2.5-8.fc18 then log in and leave karma (feedback).
mdadm-3.2.5-8.fc18 has been pushed to the Fedora 18 stable repository. If problems still persist, please make note of it in this bug report.