They're from Sequoia (one of my company's partners), and can run Palm or Windows Mobile OS's, and I'm sure also other embedded custom OS's/front ends. In the case of Apple, I'd bet either Palm or custom. As it's used purely as a POS, then the latter is most likely. I'll take a closer look next time I'm in a Store.
The basic device is similar to a ruggedized Palm, and is pretty bare but can be accessorized with an MSR, printer, barcode scanner, extra battery packs, whatever. Most models have built-in IR and WiFi. They're definitely not unique to Apple. I seem them at Hertz (and other rental agencies), and other venues where a fixed POS is too cumbersome for the task.