Those are all good points but I think it can work for Twitter because it's the only service that's: - Real time; - Accessible from anywhere; and - With a huge u

Those are all good points but I think it can work for Twitter because it's the only service that's: - Real time; - Accessible from anywhere; and - With a huge userbase already. I realize it would be a huge engineering endeavor for them. That said, as I mentioned in the article, if they don't go the "Twitter dollars" route they can go through a third party like PayPal's new open API or Amazon, which would make things easier.