Hi.
I'm using F18 upgraded from f17 with yum.
IIRC, F17 had /usr/lib/systemd/system/graphical.target.wants/<something>
to start the display manager (someone using it could clear that up).
After the upgrade that directory is empty. I had to create a symlink
from /usr/lib/systemd/system/lightdm.service to
/usr/lib/systemd/system/display-manager.service otherwise lightdm
couldn't be started, of course.
You'll have to systemctl enable lightdm.service.
See also:
http://fedoraproject.org/wiki/Features/DisplayManagerRework
Greets
Marcus