I've been playing with face recognition in iPhoto 09. It helps with tagging photos, but it wasn't clear to me exactly how to get going with it. Curiously, there is no match for "face" in the Help, so that was not, um, helpful. Fortunately, Gizmodo is helpful, with "iPhoto '09: The Definitive Review and Tip Sheet".
Now iPhoto should be done guessing photos of my oldest daughter, so I'll get back to telling it when it's right.
Update: the problem was that the help cache was out of date. Here's /how to clean it. Now the mystery isn't why there's no help, but why this problem lingers so long in MacOS./