GDM also supports autologins, but with either KDM or GDM you will end up installing a lot of dependencies, and possibly fighting GNOME or KDE from starting.
Another option is to use the rungetty package to autologin a user, and then
have the user's .bash_profile start X. To make this work you will have to disable or uninstall XDM (or any other login managers).
I have done this with some machines, and wrote a summary here: