Boot off the fedora installation CD and go into rescue mode (type 'linux rescue' at the boot splash screen I think). It should mount the system's root partition somewhere under /mnt. So lets say its under /mnt/sysimage you would then
and remove the password hash for the root entry like so:
(note that I've put junk values in for that entry - its not my real root password hash
You can do this with any LiveCD if you don't have your Fedora install CD.