The easiest way to accomplish tagging the untagged photos -- is to do it progressively. Here is how I do it.
Manually create "faces" for all the people who are important to you. Create at least 10 samples of each person. Go through the normal tagging process, but as you go, faces will undoubtedly recommend incorrect matches. This is to be expected. As opposed to double clicking and rejecting the incorrect possible match, control-click it and select "name". If you know who this person is, you can apply their pre-existing "faces" name at this point. Faces will even automatically create a new faces entry if this person does not exist in the database. This is just slightly more work than rejecting the incorrect match, but by naming it, you take it off the list of unknown matches, which lowers the number of outstanding faces to match, adds more face samples to the newly added person's confirmed recognitions list and culls the database on-the-fly. In my book, this equals tagging untagged photos as you go.
This does not guarantee that you will eventually name every person in every picture in iPhoto, but it does work extremely well -- since you know certain pictures have not come up as possible matches yet... and if you see a shot or a person you recognize -- even if they are a proposed match for someone else, use the opportunity to name them immediately and correctly -- which makes the whole recognition process work smoother -- one face at a time.