Depending on the Gnome version there are two ways to login automatically. You need to edit /etc/gdm/custom.conf and append the following section:

• This is working with Gnome version 2.22 and higher (eg. Fedora 9/10)
  

  [daemon]
  TimedLoginEnable=true
  TimedLogin=user
  TimedLoginDelay=0

• This is working with Gnome versions below 2.22 (eg. Fedora 8)
  

  [daemon]
  AutomaticLoginEnable=true
  AutomaticLogin=username