Apple released an update for iPhoto '11, version 9.0.1, that addresses a data loss problem some users have experienced when upgrading from an older version of iPhoto to iPhoto '11.
When you first open iPhoto '11, the application requires that you upgrade your existing iPhoto library to make it compatible with the new software. Some users have reported losing anywhere from a handful of images to entire libraries during this upgrade process. There have also been complaints of lost metadata and events. There are many similar problems posted in the Apple forums, but Apple says the issue is actually "extremely rare." The full iPhoto 9.0.1 update description reads, "This update addresses issues that, in extremely rare cases, could result in data loss when upgrading a library from an earlier version of iPhoto."
Along with the update, Apple has posted a support document that acknowledges the data loss problem, and gives specific recommendations for avoiding it. In addition to installing 9.0.1, the support document recommends that that all iPhoto users backup their library before attempting the move to iPhoto '11. If iPhoto '11 unexpectedly quits, Apple warns against force quitting the application during subsequent attempts at upgrading. During the upgrade process, the program may appear to be inactive for extended periods of time. This delay is especially common with large libraries, but eventually a progress bar should appear.
The upgrade is 35.4MB and can be downloaded through Software Update, or manually downloaded from the Apple site here. The full support document on the data loss issue is at support.apple.com/kb/HT4431.