Control-click on the iPhoto Library file and choose Show Package Contents from the contextual popup menu. In the resulting Finder window, look for two files: face_blob.db and face.db. Select them both and copy them to another location on your hard drive. Now, if your Faces database becomes corrupted, you can pull out the old files and replace them with the ones you have backed up.
Keep in mind that you can't transfer these database files to other iPhoto libraries. And if your hard drive crashes, your backup won't be of much use. iPhoto '09 is fairly temperamental when it comes to reconstituting its Faces corkboard; not even the Burn command within iPhoto preserves these connections. The only surefire method I've found for preserving the entire library--including all of the work you've invested--is to copy the entire iPhoto library to a separate hard drive. But if the database inside of iPhoto goes bonkers on you, you may be able to restore your sanity by replacing the corrupted database files with the ones you've backed up.
By the way, if you ever want to start fresh with your Faces database, you can also use this trick to remove the database files from your iPhoto Library. Quit iPhoto, remove face_blob.db and face.db from the package contents, and restart the application. Doing so will force iPhoto to start anew with its face detection process--useful if you discover you have a lot of mistyped names, wrong identities, or regret adding pet names.
[Senior Contributor Derrick Story teaches iPhoto on Lynda.com and runs a virtual camera club at http://thedigitalstory.com.]
This story, "Back up IPhoto's Faces Database" was originally published by Macworld.