EDIT: The update for kdebase-workspace (4:4.6.2a-0ubuntu1~maverick1~ppa3) released on 04/10/2011 solves this bug.
Instead of shutting down the computer you’re greeted with the login prompt of tty1. This behavior is caused by a bug of KDM’s default settings. It uses a command line switch for shutdown which isn’t known by Ubuntu/Debian linux.
To solve this issue go to System Settings->Login Screen. Got to the tab „Shutdown“ and change the command to shutdown your computer to
/sbin/shutdown -h now
Save and log off, restart X-Server, log in again and everything should work like it did before.