One possible scenario involves Apple leveraging the e-commerce potential of the iPhone by allowing users to pay for real-world items in-store using their mobile devices in combination with their iTunes account. If an iPhone could replace cards, wallet and cash, that would definitely become a selling point for many.
Oh yes, most definitely. If I could pay for shopping / bus and train fares / etc. wirelessly with just my iPhone, that would be awesome. Very futuristic.