Mobile Payments a replacement for cash?

It strikes me as obvious that we really need mobile instant banking. People live their lives at quite a pace, purchases will be made and completed in minutes, but then people have to wait for the money and their bank statement through the post. It would be so simple to instantly send and receive payments via mobile number. Conventional functions like banking need to catch up with the way people live and work in the 21st century! It could even form the basis of a replacement for the age old system of carrying cash around!