Cash, Card, or Phone?

Do you use bus card and other pre-paid cards for transportation? Ten years ago, JR started issuing iO cards. Then Metro cards and bus cards followed. At first I thought it was pretty convenient, but I found a problem. The fact is that these days, you probably don't use your phone cards often, because most of us have a mobile. Well, there was a news item recently that mobile phone users will be able to buy credits on their cell phones, that will act like tickets/cards for your daily transportation needs. Every time you exit a ticket gate, even without showing your phone, the ticket machine will let you through and record the transaction via your cell phone -- which you can just simply leave in your bag.
As some of you might have noticed, Japanese are not used to using credit cards, but instead tend to overdepend on cash. Debit cards certainly haven't worked well here, and the preference for chash may be one of the reasons. Maybe the banks should try to add debit card functions to mobile phones, so the consumer doesn't have to carry around so many cards...