Better idea: as soon as I get a call from an unknown number, do a search for the number in facebook to see if you can find more information, and pop up that information on the phone before it even starts ringing.<p>There might be a problem of spam whereby one person creates a fake persona with their phone number and pretends to be someone else, but I think facebook could solve that problem.
It's always been the case with the Facebook app pre-iOS6 that after syncing contacts (which uploads all your local phone numbers to Facebook) any Facebook profile associated with one of those numbers would appear in your address book on the Facebook site.