From Macintouch. This matches my experience w/ OS X debugging. Deleting prefs and caches is much more useful than the usual recommendation to "repair permissions".
Of course iPhoto should manage problems with cache or prefs far more gracefully that it does. That's another story.iPhoto (Part 12)iPhoto 4 Freeze
My copy of iPhoto 4 got completely stuck yesterday, just out of the blue. I had last used it a few days (and quit) without any problems. Now, the spinning disk came up a few seconds after start-up and I had to force-quit the app. This was absolutely reproducible. Trashing the preferences caused iPhoto to open with the default settings (window size, size of left bar) but again the spinning disk appeared after a few seconds.
What fixed it was cleaning the font caches (AKA user caches) with Cocktail.