Following on the heels of the not a face hint, here's another iPhoto '09 face matching trick I've noticed.
When looking for matches for a person, iPhoto will present a list of faces that it thinks match. If you click on Confirm Name, then you can accept or reject names as you would expect (using clicks, drags, option-drags, etc.). But as of iPhoto '09 8.0.2, you can also control- (or right-) click on the face image itself to show a contextual menu that lets you not just state that a face is right (Correct) or wrong (Reject), but to actually identify that face (Name).
With this change, my workflow for tagging faces has gotten simpler. First, I option-drag to reject all the faces iPhoto suggests. Then, I click or click-drag to confirm the guesses that iPhoto got right, or I right-click to specify the known ones that iPhoto got wrong (or mark non-faces as 'Not a face,' or frequently-suggested strangers as 'Stranger'). That way, I minimize seeing the same faces over and over, because each identifiable face only has to be scanned (by me) once.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090428103427234