Well ... does the Apple site say anything?
Aperture takes Faces further.
Aperture 3 expands on the power of Faces in iPhoto. Not only can you view the people you name across the entire library, you can now see them in individual projects. And the new Show Unnamed Faces pane displays all the detected-but-not-yet-named faces in a project to help you easily add names
I don't use iPhoto, so I can't comment on whether this has changed in the interim for iPhoto.
I guess if I wasn't so lazy I could have clicked once more and gotten the answer myself.
I checked iPhoto '11 and this statement is still true. In iPhoto '11 (version 9.x?) you can only view Faces by Library, not Event. (or Project as is the case with Aperture)
I'm not sure how helpful the Unnamed Faces panel is. It seems you can only view a film strip worth of images. I don't think you can browse unarmed faces.
I don't think you can browse unarmed faces.
You can use the Filter Rule "Face" set to "is unnamed" with any container(s) selected.
I do my Faces by Project (as needed). After Aperture's first pass, I filter for "Face" "is not detected" and force a rescan (as needed).
Imho (again: I don't use iPhoto) this implementation is useful.