If iPhoto is slow to launch on your machine—as in taking over a minute to load, including visits from the spinning beach ball—here’s a possible fix that also happens to be a simple one. Once the application finally launches, open its Preferences by hitting Command-, (that's Command-comma), then click on Sharing.
In Sharing, see if either the “Look for shared photos” or “Share my photos” boxes are checked. If they are, and assuming you’re not using these services, uncheck those boxes. If you are using the sharing features, you may still be able to remove the check mark from one box—if you never access other shared libraries from the current machine, remove the check mark from the “Look for shared photos” option, for example.
When I posted this tip (from an anonymous contributor) on Mac OS X Hints, I was skeptical that it would make any difference, as it didn’t in my testing. However, many users have commented that it did, in fact, greatly reduce their iPhoto launch times. So if you’ve got a slow-to-load iPhoto, and you don’t use the sharing features, check to make sure they’re disabled in the program’s preferences.