I recently upgraded to Tiger using the Erase and Install method that Joe Kissell recommends in Take Control of Upgrading to Tiger; I wanted the cleanest start with the new operating system. Most things went fine, except for launching iPhoto 5.0.2 afterwards; it always hung with the spinning pizza of death. I eventually solved the problem, but to give you an idea how I troubleshoot - along with the eventual solution - here's what I tried:
I rebuilt the iPhoto Library by holding down Command-Option at launch. Unfortunately, that didn't help. The goal here was to see if some sort of repairable corruption in my iPhoto Library folder was causing the problem.
I switched to another iPhoto Library by holding down Option at launch and choosing a different iPhoto Library folder. This didn't help, but eliminated the possibility of the problem being in my main iPhoto Library folder.
I created a new, empty iPhoto Library folder (again started by holding down Option at launch). The goal here was to see if there could be some problem related to an iPhoto Library folder that had been used in Panther.
I moved the com.apple.iphoto.plist file from the Preferences folder to the Desktop to see if corruption in that file might be the culprit.
I opened iPhoto's Info window, and in the Plug-ins section, I deleted all third-party plug-ins that I'd installed. Most were turned off anyway, but I wanted to make sure none of them could be the cause of the freezes.
With those five attempts under my belt, I figured the problem was most likely related to the iPhoto application itself (since I'd eliminated everything else I could think of). So I trashed the iPhoto application and reinstalled it from the iLife '05 DVD. On the next launch, I told iPhoto to create yet another empty iPhoto Library, and for the first time since installing Tiger, it continued to run. I then updated it to 5.0.2 using iPhoto's Check for Update feature and Software Update, and after that, I was able to load all my iPhoto Library folders.
Since the disk had just been reformatted, I didn't suspect any sort of directory corruption, but if reinstalling hadn't worked, I would have used Disk Utility to repair permissions and then, if that didn't work, to repair the disk itself.
Although I was annoyed at having to work through iPhoto's freezes, I knew that I had two current backups of my entire hard disk, so I wasn't particularly concerned about losing any data.