Someone had a similar problem a little while ago with a more current iMac. For what it's worth, here's that thread: http://www.ifixit.com/Answers/View/1470/iMac+turns+on+and+off+immediately.+Any+ideas
That person believes it was a hard drive problem since he/she was able to start up the computer from an installation DVD. But that person doesn't mention anything about a power outage as you have...