Search for untagged photos.

Not directly. However you can go to the Faces mode, select all faces and hit the Smart Album button. In the resulting smart album assign a keyword to all photos like "faces". Then create a smart album whose criteria is Keyword is not "faces". Then as you assign new faces to the photos you'll need to go back to the smart album for faces, select all and reassign the "faces" keyword. I know what you're thinking but until Apple provides a new feature that's what we're saddled with.

Be sure to request that new feature via http://www.apple.com/feedback/iphoto.html