hate to piggy back this thread, but i am having a similar problem with my old monitor, 17" viewsonic crt.
a few weeks ago, it started waking itself at night, programmed to shut itself off after 10 mins idle. it was perfectly functional but refused to stay down if the computer was on. i figured it was a software issue, something i had running in the backround, but when i got my new LCD it stopped.
i have since sold it to a guy i know and he reported the same problem on his system and his son told me yesterday that while he was playing a game it just powered itself off, which it never did for me and hasn't done since. once he re-booted it was fine.
i re-installed the drivers on his system today so i'll see how that goes, but i wonder if it's an aging/dying monitor issue.