I know that the one drawback with iPhoto 2009 as been the inability to simply find pictures that have contain unidentified faces… So I was glad to see from this Mac OS X hints entry, that this process has been simplified…
Create an ‘unnamed faces’ Smart Album in iPhoto ’09 While iPhoto 09’s new facial recognition is pretty good, it’s not perfect. You can help it out by creating a Smart Album of unrecognized faces, and then using that to manually tag faces. With the release of iPhoto 8.0.2, Apple appears to have made this process much easier.

Create a Smart Album with the following criteria: Face — is — unnamed.

You can now tag faces in the photos in this Smart Album, and as long as there’s at least one ‘unnamed’ face in the photo, it will remain in the Smart Album (negating the need for a second album, as explained in the first hint’s comments. “