I recently installed 12.04, but I had the same problem in 11.10. When I watch a video, VLC suppresses the monitor being turned off. In older (pre-Unity) Ubuntu versions, this would only be in effect while the video was playing. Now VLC is disabling the turning off of the monitor regardless of if it is playing or not.
Is there a way to fix this? Or a less quirky shell? Or a video player that doesn't exhibit this behaviour?