The latest version of iPhoto brings with it a number of desirable features, and many people have jumped on the opportunity to install the software. Though it has worked fine for many, there are a number of people who have had a few problems. One of these problems is the program having difficulty when converting the iPhoto '09 library for use with the new version, where the program will hang on a gray screen with a spinning progress indicator.
There are a couple of reasons why this might occur. MacFixIt reader David S. wrote in mentioning that one possibility is that some old keepsake items like photo book calendars may not be compatible with the new version.
Before upgrading, open iPhoto '09 and use it to delete the "Keepsake" items from the left sidebar. If you no longer have iPhoto '09, you can install it with your old iLife application discs. After the keepsake items are deleted, launch iPhoto '11 again and try importing and converting the photo database.
If this still does not work, you can try manually delete the projects database in the iPhoto library along with some relevant files, and then retry the import. To do this, go to the library file on disk, right-click it and select "Show Package Contents." Then locate and remove the following items:
Deleting these items should not affect any photos in the library, and the next import should recreate the projects database.
Among other problems are people finding that some or all of their photos have disappeared after the upgrade. It is likely the upgrade has not deleted the photos themselves, but instead the library no longer properly points to the photo files.
If this happens to you, try repairing and rebuilding your iPhoto library by holding down the Command and Option keys when launching the program. You will be shown a menu with options to rebuild thumbnails, check file permissions, and rebuild databases, but more relevantly have an option to recover orphaned photos that are in the iPhoto library folder. If photos exist in this folder that are not showing in the library (and therefore not showing in iPhoto), then this should find and re-implement them in the library.
Lastly, a common error that OS X programs experience is preference file corruption, which can especially happen during product upgrades. If the program is showing odd behavior, running slowly, or especially not saving settings, then go to the /username/Library/Preferences/ folder and remove the file "com.apple.iPhoto.plist." Then relaunch iPhoto and hopefully the program will begin to behave properly.
If you have not already upgraded to iPhoto '11 and are considering the move, be sure you fully back up your iPhoto library first. Apple's Time Machine backup can do this, but for added security you might still copy the library to an external drive or another computer before installing the new version. Be sure to run the cleaning and rebuilding routines for iPhoto '09, as well as run a permissions fix on your computer's hard drive using Disk Utility before installing iPhoto '11.
Additionally, if you have any third-party plug-ins or other add-ons for iPhoto, be sure to remove them before upgrading, since incompatibilities with them may adversely affect the new version of the program.