I had the same problem but after installing KDE 4.6

At the same time I started receiving some audio problem :
"The audio playback device HDA Intel (AL880 Analog) doesn't work.
Falling back to ."

...and other problems with the mp3 support.

Don't know the reason but now I can logout and shutdown normally after I disabled the sound notifications
Personal Settings->Application and System Notifications->Player Settings->No audio Output

Hope it can help.