when your iPhone rings and four other iPhone owners on the train reach for their iPhones b/c they have the same ringtone! Has this happened to you recently? 
I turned a bunch of Garage Band and Soundtrack Pro loops into ringtones.
Nearly everyone in my Address Book have their own unique ring, and my 'default' ring is also something I added.
So I have never been confused by anyone else's phone!
Are you never confused by your own ring tone? You either have a very small address book or you must forget some of the tones are your phone.