You might try rebuilding your iPhoto library.
http://www.macosx.com/content/faq.php/q13024/Iphoto-Wont-Open.html
To get iPhoto to rebuild itself, you quit iPhoto, then launch it while holding down:
* the "shift" and "option" keys (for iPhoto 4)
or
the
* the "option" and "command" keys...