Apple stores print receipts from their iPod touch credit card readers. They probably use a custom solution with a server backend.
Square has a few wireless receipt printers it can work with.
I haven't yet seen a receipt printer that is airprint compatible though, allowing more apps to use it. While credit card acceptance and sales apps would obviously benefit, I can imagine a number of other uses for a small printer that prints bits of disposable information. The daily forecast, sudoku puzzles, today's schedule, todo lists, etc, for those who have to share their iPad with a spouse or children, for instance.
Are there air print receipt printers, or is there an easy way to set up a receipt printer, perhaps with an airprint server (device or software on the mac) that would allow this?