I have a Fedora box running and I sort of screwed up the GDM so I was left without a display. Since the Fedora 13 (Goddard) was released recently, I decided to upgrade, from the command line. It kept giving me the error that complained, "Can't open display".
I finally figured out that I needed preupgrade-cli, as specified here: http://fedoraproject.org/wiki/PreUpgrade
The syntax, from the command line is this (as root):
preupgrade-cli "Fedora 13 (Goddard)"
Note the quotes surrounding the argument - you need those to specify the string for the version you want to upgrade to.