How long till we can buy everything with our phones?

When will we be able to pay for everything with our mobile phones?

I'd love to be able to do that. The store person would tell you the cost, that'd flash up on your phone's screen, you could accept it and it'd all be done. You'd get an electronic recipt, and you could automatically feed the purchase into whatever software you like to keep track of your expenses, which could streamline the process of budgeting.

The potential benefits are pretty obvious, and I know there must be a lot of people working on this kind of thing, so I'm wondering what sorts of barriers there are to getting it out there. It might involve technical issues, though I suspect it might have more to do with things like cost and business models. And given that, how long might it take until the technology is out there?