Since BT5 won't use the SystemV system anymore but upstart you have to at first run:
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:
... script export XORGCONFIG exec gdm-binary $CONFIG_FILE end script
Should do the trick. For KDE it should be
exec kdm (instead of gdm-binary $CONFIG_FILE)
Source: http://www.backtrack-linux.org/forums/showthread.php?t=42046