Comment 21 for bug 403291

So, the idea is to call /usr/lib/gdm-set-default-session to properly define default session (it handles cases where gdm-daemon is up or down).

Here is what has been done in xubuntu-default-session:
- removal of dpkg-divert for gnome.desktop in postinst (as it was only internal to karmic development cycle)

- add to postinst in upgade|configure:
if [ -x /usr/lib/gdm/gdm-set-default-session ] ; then
   /usr/lib/gdm/gdm-set-default-session --keep-old xfce || true
fi
-> this update the default session if and only if no previous session is definied (this handle both the case of installing multiple derivates on the same system and having only one derivate which will be the default). If you want to add it and erase previous default session (which is none by default), remove --keep-old option.
Of course, you remplace xfce with your <session>.desktop without ending it with .desktop.

- add to postrm remove|purge:
if [ -x /usr/lib/gdm/gdm-set-default-session ] ; then
   /usr/lib/gdm/gdm-set-default-session --remove xfce || true
fi
-> this remove your session if and only if your session was the default one.