Geek Sugar today profiles using the iPad as a Point of Sale system at Sightglass Coffee Shop in San Francisco. They use the Square headphone jack credit card reader (white, top left) and also have built a pretty awesome rotating enclosure for the device.
I think this type of use for the iPad will be pretty popular. The bigger-than iPhone size makes it easy to operate like a traditional cash register and its price at $500 is competitive with other POS systems. Square is demonstrated in the YouTube video below: