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).
Fixed in dhcp-4.2.4-13.P1.fc18.
dhcp-4.2.4-15.P2.fc18 has been submitted as an update for Fedora 18. https://admin.fedoraproject.org/updates/dhcp-4.2.4-15.P2.fc18
dhcp-4.2.4-15.P2.fc18 has been pushed to the Fedora 18 stable repository. If problems still persist, please make note of it in this bug report.