Square have disrupted an enormous market space. Paypal and Verifone are following in their footsteps with nothing new. That said clearly this huge space will be filled with many players. POS or point of sale has many many vertical markets and these big players with their simple apps cant possibly fill it. To that end sail is promising an API to allow smaller start-ups to use the simpleness of the ease of sign up to credit card services, as this clearly is what is fueling this trend. Many new stores and business's like the ease of entry that square offers. But clearly once on board they see the need for more defined apps that fit their model and vertical market.
Disclosure: I work for Tillify.com. We build mobile payment apps for Android, IPad, Mac and PC. Like square, Paypal and sail we allow you to accept payment and run your business using a mobile device.