Apple has released iPhoto 9.0.1, which “addresses issues that, in extremely rare cases, could result in data loss when upgrading a library from an earlier version of iPhoto.” Users should install this update before updating to iPhoto ’11.
This issue can result in a situation in which photo libraries are blank or missing after the update to iPhoto ’11, and all of the contained images are apparently deleted
A corresponding Apple knowledge base article states:
“If iPhoto unexpectedly quits during the update process, first confirm that you have installed the iPhoto 9.0.1 software update before attempting to upgrade your library again If the software unexpectedly quits, iPhoto may on subsequent launches appear to stop responding during the upgrade process. Customers are advised to let the process continue until the upgrade progress bar is displayed.
“If you have force quit iPhoto ’11 during a library upgrade, you should not attempt to open iPhoto ’11 or upgrade the library again without first installing the iPhoto 9.0.1 Update. Important: Attempting an update after a force quit may cause data loss.”