Actually, I've seen this happen with mdkkdm as well lately. I was waiting to get a stable cooker (with an installable KDE :-) ) to report it, though. The symptom is the same: you choose either shutdown or reboot from GNOME, and mdkkdm comes back up with the login prompt.Yes, I saw that yesterday on my test system (it is not related to gdm).. I'll investigate monday.
You can fill a bug against gnome-session so I won't forget about it..
I pointed out bug 3584 because the visual symptom is the same if you don't actually CTRL-ALT-F1 to the console screen and notice that shutdown is in progress, and a shutdown usually takes longer than it take people (me, anyway) to decide that my shutdown request didn't work and redo it from mdkkdm or gdm.
