I've got a couple of MythTV frontend machines where its very useful to be able to switch it on and have it login without prompting for a password then auto launch MythTV.
In Fedora 15 / 16 and 17 this seems to be done a little differently and took me a while to find out what needed changed in the config. Basically you need to add the following to the [daemon] section of /etc/gdm/custom.conf
AutomaticLoginEnable=true AutomaticLogin=[YOUR USER] TimedLoginDelay=5
So an extract from my own /etc/gdm/custom.conf
[daemon] AutomaticLoginEnable=true AutomaticLogin=m00nie TimedLoginDelay=5
Now just reboot and you should auto login with no prompting for passwords :)
(Just to note this is a security risk)