Since quite some people asked I played around a bit and here is how it is working:

Since BT5 won't use the SystemV system anymore but upstart you have to at first run
Code:
apt-get install kdm
or
apt-get install gdm
Afterwards edit the file /etc/init/kdm.conf or /etc/init/gdm.conf depending on what you decided to install.

Edit the contents so that it reads:
Code:
...
script
      export XORGCONFIG

      exec gdm-binary $CONFIG_FILE
end script
Should do the trick. For KDE it should be

Code:
exec kdm (instead of gdm-binary $CONFIG_FILE)