Cue a market for matching disembodied fingers with the appropriate iPhone.
But seriously: I really hope they require a thumb-print accompanied by a pulse (is that even possible?). The other thing is, I wonder what the override process will be in the event that users want to sell the unit once they're ready to upgrade. I'm sure there will be one, but wouldn't it be great to get your newsed phone in the mail only to realize the dumb previous-owner forgot to scan his/her thumb to unlock it for your new thumb? I dunno...I guess there's so many issues with this that I don't particularly see this as a step forward. What do you think?