What square has done is great. I have seen another start-up at a recent event that was showcasing a product that does the same thing. It has some advantages that might better suit bigger retailers as the square app is pretty feeble in terms of a pos system. That said it is beautiful, Tillify multi-tasked as it allowed for multiple operators to run multiple sales at once - it was really sweet.
The start-up was Tillify.com and they are allowing vendors to use their own merchant account and also use PC's or Android Tablets as well as IPads.
It looked very well thought out and like a serious contender in this field.