Now here's a problem I've never had before - my Lenovo Thinkpad T500 won't turn off. If I choose "Shutdown" from the menu, or enter "sudo shutdown -h now" at a console, the machine obligingly shuts down - then immediately reboots and starts again. The only way I can get it to actually turn off is to remove the battery and then re-insert it. This seems to break the reboot cycle. But why is the laptop rebooting instread of shutting down, and how do I get it to behave properly?