Like other proofs-of-concept with Fujitsu's PalmSecure technology, you don't actually have to lay yourhand on the sensor to make it work -- hovering above it will work just fine.
When you watch Jony Ive enthuse about the chamfers and the microns in the official Apple video that was shown at the launch event on Wednesday (see above), you are hearing the implicit backstory of not just how the device feels in your hand but how it makes you feel about yourself.