Apple has updated its popular iPhoto program to version 9.1.3, which addresses a problem where merged photo events could split back up again after being synced to iOS devices. The update is approximately 106MB and requires OS X 10.6.6 or later to install. It should be available through Software Update for those who have iPhoto already installed, but it can also be downloaded and applied manually from the iPhoto 9.1.3 update Web page.
Be sure to back up your iPhoto libraries before applying this update. If you have Time Machine set up, you can select Back Up Now from the Time Machine menu to ensure that a backup is done, or if you do not then you can copy the iPhoto library to an external drive via the Finder. This update does not require you to restart your computer, so it can be applied without disrupting your workflow.