My old Thinkpad had a fingerprint sensor. I kinda liked it, in terms of sitting in a public place like an airport and not worrying about shoulder surfing. If Apple were to have a biometric sensor, maybe they'd do it in some other sort of cool fashion, like face recognition via the camera. But then they'd have to come up with some kind of make sure the person is really not just a photograph detector. Hmm... Maybe it asks you to blink your eyes in some pattern?