While iPhoto 09's new facial recognition is pretty good, it's not perfect. You can help it out by creating a smart album and manually tagging faces. To find a list of photos of everyone who hasn't been tagged by their face yet, simply create a new Smart Album with the following criteria:
Name -- does not contain -- (leave the field blank)
You can then quickly tag a bunch of photos, and iPhoto should begin recommending a few more.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090128011708843