I doubt this is going to ever happen. At least not for awhile. People borrow each others phones all the time. Then apple will have to make a way to be able to cancel it if something goes wrong. You won't be able to unlock it. Therefore, someone will find a way to do it and be able to crack it. Also, a friend takes your phone for a bit. They can't use it... It would just get annoying. + the technology is multi touch, but not finger print sensitive. Also, you can't even use a stylus which is muuuuch thicker then the line on a finger.
Not for a loooooooooooooooooong time.