Apple has released a software update for iPhoto. The iLife 09 update addresses a few bugs dealing with Face recognition performance and accuracy.
Apple has released a statement to provide extra support for getting the face recognition working properly:
Important: If you imported photos of people while using iPhoto 8.1, there’s an extra step you should take after installing the 8.1.1 update. Select all those photos, Control-click to open the contextual menu, and then choose Detect Missing Faces. This will redetect the faces in those photos and correct any face recognition issues introduced in iPhoto 8.1. You should not quit iPhoto during the Detect Missing Faces process.
If you have high iLife, make sure to update right away.