I'd imagine capacitive buttons. iOS doesn't work without a home button. And as for the obtuse undiscoverable gesture concepts that go along with the no-button ideas: Apple has to be the last company that I would expect to ship a phone that cannot be taken from any state to making a phone call while held in a single hand.
Capacitive buttons create accessibility and usability problems for all users. They look great in demos and quick previews. They're crap for use.