Can you access your vehicle through these methods?
No for Roadster and unknown for Model S.
Would it be cool? Certainly, as the promise of NFC, as it currently exists in Japan, is that your smartphone becomes your credit card, would be even better if it could become your car key as well.
Would it be "better" than a key fob? Maybe not, since NFC needs to be within a few inches of the base. So I've added the extra step of taking out the phone and then need to activate the NFC application. I'd rather carry around a key in my pocket, IMHO.