To go back to the subject, gdm and kdm allow you to autologon an user. The next step would be to run vdr from .xsession for ex.
I use kdm and kde with autologon.. Starting "needed" stuff (like vdr-sxfe) in directory ."kde/Autostart/". vdr-sxfe starts at first desktop in fullscreen mode and rest of programs (like web browser and few xTerms) at other virtual desktops.. So, when I need to "do something" with that computer (hey, it's HTPC not only VDR) I just "jump" to some virtual desktop to do.. When I jump back to desktop 1, I can continue with TV..