iOS or Airprint receipt printers? 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?
The Apple Store doesn’t list any AirPrint-compatible receipt printers.

However, the AirPrint Activator (an app for your Mac) allows you to use any shared printer via AirPrint.