I just made an DVD based update from Fedora 10 to 11. After the first boot I was able to login and start an update of all packages (about 2GB).
But at the next boot sequence the pylmouth screen hanged and GDM did not start up. Interestingly enough if I switched fast enough to the boot console with ESC, GDM startet flawless.
After investigating for a while I found out that the following line in /etc/X11/prefdm caused the hang problem:
/usr/bin/plymouth quit $quit_arg
I fixed (by meaning of workaround) the problem by completely disabling starting plymouthd at all. To do so just remove the rhgb option in the appropriate kernel line in /boot/grub/grub.conf.