Apple has released iPhoto 9.0.1 via Software Update. The update addresses issues that, in extremely rare cases, could result in data loss when upgrading a library from an earlier version of iPhoto. Indeed, many of our readers have complained of such issues and our own Mel Martin wrote about them in his review of iPhoto '11.
The update weighs in at 33.87 MB and requires iLife '11 and Mac OS X 10.6.3 or later. You can also download the update here and read Apple's knowledgebase article here.