Have a look at the thread on Flux. They are working on a system exactly like what you’re talking about, and have asked for feedback on what sort of features people would find useful.
There was also a link to an article about Flux posted in the Competitor Update thread.
Edit: I think the advantage of having a separate company develop this system, is they are more likely to succeed in getting POS developers/retailers on board, as it will be seen as attractive to a wider range of people, not just those with one bank.