Square was my first thought, too. Yes, they could most certainly take on MC/Visa/Amex/etc. Maybe that is why Visa invested in Square? You have to get the POS units into retailer's hands. Then, at the same time, you have to get consumers using it. Where square differentiates themselves from the competition is that they make it very easy for anyone to become a POS node.

Now, the difference between Square and the CC companies is that square is really a gateway to those CC systems and not a credit provider themsleves... BUT once they have traction with POS merchants and acceptance by consumers could they start their own credit department? Sure. Would they want to? That is another debate. I'd wager it's much easier and still very profitable to be a gateway taking a small slice off each transaction than it is to be the credit provider.