I'm new to Fedora and I'm using F17/Xfce.
One of the first things I did to 'acclimatise' was remove selinux. Then after I got more familiarised with the system I decided to install xguest, which brings selinux back.
xguest never appeared as an option in GDM and I thought it was because I had selinux disabled (xguest requires selinux in enforcing mode to work).
I did all this in a 'not in a hurry' pace. So one day I installed xguest/selinux, other day I set selinux to permissive mode (and made sure the relabelling was all fine) and today I set it to enforcing mode.
But xguest still doesn't show up as an option in GDM.