How do I set Xfce as my gdm default?

[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