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.]

