For compact payment ‘protocols’ also see:
TipJoy: Twitter Startup Fails Despite Major Blessings
It should be recognised that the mobile phone platform does have a significant advantage compared to the web/Internet of anonymous devices.
However, I think payment is over-simplified when it is posed as the problem: “How can A transfer funds of $X to B?”.
I suspect that the more sophisticated problem of “How can A and B express a public record of their agreement to make the equitable exchange Xa for Yb?” might be more tractable.
The first is concerned with manipulating external funds via browser/website/paypal. The second is concerned with establishing for the public record that A and B agreed to make an exchange, probably monetary.