It suddenly took was taking iPhoto over a minute to load on my machine, with the spinning wheel visible during this whole time. This went on for days. I read various solutions online, and tried turning off program options.
Eventually, I found a solution to the problem. In iPhoto's Preferences, on the Sharing tab,I disabled both sharing options (looking for and sharing). iPhoto then loaded nearly instantly.
[robg adds: I don't know if this is a widespread issue; I have sharing enabled on my two main machines here, and both seem to load iPhoto with no change in launch time based on this setting. I suspect a broader network issue is behind the problem, but if you're trying to troubleshoot slow iPhoto launch times, this may be worth trying.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20091228062404250