How do I set default gdm session?

[Todd And Margo Chester]

Hi All,

I just set up a computer with Scientific Linux 6.1, using
the default "Workstation".  Then I installed Xfce.

Question, how do I change my gdm default from gnome to Xfce?

      /etc/gdm/custom.conf
      [daemon]
      DefaultSession=xfce.desktop

Does not work.  What am I doing wrong?

Many thanks,
-T