I have been experiencing an infinite shutdown process (or at least 45 min - 1 hour before I force shutdown) at times since my upgrade to 10.2.3 on my Dual USB iBook.
This has happened twice and only seems to happen when I have been working in iMovie.
The process gets to the final shutdown screen and the dash-wheel just keeps on spinning. It even gets to the point where this screen dims itsself, which I assume is just the powersaver function, but never shuts down.
It was a clean install into Jag and this never happened in 10.1.x