Same story here 😟
I just upgraded from OS X 10.4.8 to 10.6 yesterday with the Mac Box Set including iLife '09. I have 9,500 photos. It took 90 (?) minutes for the initial Face scan. I started with a half-dozen people and tagged a few photos. It couldn't find a single match, the wheel just kept spinning, even after leaving it alone for hours.
I did the following:
1. Deleted the face.db and face_blob.db (as other suggested).
2. Made sure to remove Movie files from iPhoto (as suggested at
http://www.somelifeblog.com/2009/02/iphoto-09-faces-crashes-when-clicking.html).
3. Restarted many times. Allowed iPhoto Face scan to run again (took 1+ hour).
4. I did NOT try to rebuild the iPhoto library as some have suggested.
At first I made some small breakthrough. Out of the 8 people I added, finally I got one of them (my 6-month-old son) to start seeing some matches. iPhoto actually did a pretty good job of matching although there were some hilarious mistakes (my son is pale white and bald and it matched him with a photo of a 10-year-old black boy and one of my adult friends with a thick mustache and black curly hair).
Anyway, I found that quitting and restarting iPhoto, it would sometimes match my son again, and sometimes it would just spin.
It could not make matches with any of the other faces. I even left the computer on all night, no progress. Here I am the next morning, and Faces recognition won't work at all.
Sooo, I think the only interesting note I have to add to the conversation is that it did just barely work for me, but just a couple of times, and just on a single Face. Disappointing. If you think that going from 10.5 to 10.6 will help, it didn't work for me. I never tried iLife 09 on my older 10.4 OS.