Rob Griffiths provides a helpful tip on using Smart Albums in iPhoto ’09 to reduce the gargantuan load of tagging faces:
You can create a similar Smart Album for photos with no face entries, but there are a couple of additional steps needed to make it work better. As a first step, create a new Smart Album with two conditions (name it something like Needs a Face – Master), and with the Match pop-up menu set to All. The first condition should be Name : Does Not Contain, and then leave the text box empty—this basically finds all photos that don’t have any identified faces.
via Use Smart Albums to help set iPhoto ’09 Faces and Places | Photography | Mac OS X Hints | Macworld.