I'm having this problem also. I have a Dell Latitude D630, and started having this issue shortly after install (maybe a week after).
I tried turning on the laptop, but not logging in. Then I clicked the shut down icon, and the computer shut down with no problem. I also enabled "Guest" profile on the computer so I could see if it's having the same problem outside of my profile. It is. I can switch users, turn off the computer, outside of either profile with no problem (as long as I don't log in, no problem shutting down the system).
Obviously, the issue isn't with shutting down, but it's having a problem logging off only, so you don't get a chance to get to where the system shuts down.
My computer does this every time, and will not (log off so I can) shut down at all, until I either force quit (hold the power button down) or remove the battery.
Has anyone found a solution for this issue?
Thanks in advance.