I have ubuntu 13.04 64 bit on Thinkpad T530. In my settings->'Brighness and Lock' I have disabled turning off the screen for inactivity. And when I am running without docking station this is how it behaves.
When I am plugged in docking station, I have an external monitor via VGA port on docking station. And for some reason after either 10 or 15 minutes of mouse/keyboard inactivity both screens flip to text mode with black background,and shows sequence of @ chars in the top left corner. If I press keyboard, move mouse it stays in this text mode and I have to reboot.
I had gnome-screensaver installed. After uninstalling it I still have the same problem. There are no other screensavers installed. Somehow I think it is related to power management rather than screensaver.