i had the same exact problem, this is the reply i go......hope it helps, let me know if you figure it out, thank you.
Your best bet is to drag everything associated with iPhoto (especially the app and pref file) to the trash then reinstall it from your MacOSX Installer disc. There's a utility will let you do this but I've forgotten its name. If you have the separate iPhoto installer then simply use that. Then drag your photos back in.