You can do this easily with Square Cash and Venmo already.

Yes, but you asked about scenarios where I am pretty sure the other party has an iDevice. I am not in any way saying that Apple's solution would be superior to existing solutions.

Speaking as a Canadian, I can already use email to transfer money to almost any other Canadian, because our cartel of banks use a common system.

As someone who has used both extensively, they both could use improvement.

Venmo is a holding service, requiring an extra step to actually get your money.

Square Cash, while easy and directly person-to-person, is slow.

Fix either of these issues and I'll convert.