In testing f15-alpha, looks like gnome3 user switching + kdm doesn't work. I get an error:
Unable to start new display
the name org.gnome.DisplayManager was not provided by any .service files
I'm hoping that this can be made to work through some combination of adjusting gnome3 and kdm to be compatible.
I'm not aware of anything having changed. But gdm is almost certainly the wrong component to have this assigned to, it should be something like gnome-shell or gnome-session, not sure.