The only issue might be privacy. Your phone book is full of people, 90+% of whom probably do not have an iPhone. On Facebook, every one of your Facebook contacts has clearly already divulged to you that they are on Facebook.
Still, I imagine if Apple made an opt-out method of automatically friending all of your email contacts who have iPhones (or at least an opt-out method of seeing them) it would be popular.
Still, I imagine if Apple made an opt-out method of automatically friending all of your email contacts who have iPhones (or at least an opt-out method of seeing them) it would be popular.