I did have this problem initially right after flashing to ICS. I also thought it was a stuck power button due to the Commuter case. But that wasn't the case.
You could try to select reboot from the menu. Then after the reboot, select power off to see if it'll power down properly. Maybe some bug in the menu thinking you hit reset instead of power off.
I can't remember if that fixed it as I hard wiped to factory defaults twice already. Once after the update to play with it and see how it is; then second time prior to actually restoring my data.
One of these solutions fixed it for me as I haven't seen it since.