To do this you need to edit /etc/gdm/gdm.conf or /etc/kdm/kdm.conf or /etc/xdm/xdm.conf. In the gdm configurations files there is a line labeled like, "AutomaticLoginEnable=false". Change the false value to true. Later in the file there will be a line labeled like, "LocalNoPasswordUsers=". Put the user name you wish to have login automatically. Such as "LocalNoPasswordUsers=mythtv". Next time you boot your computer it will login as this user. If you are using kdm or xdm instead of gdm, the lines that need to be changed will not be labeled the same. If you read through the kdm or xdm configuration file you should see lines that mean the same thing.
