Since computers started as calculators, it's not such a stretch. It's just an especially great development for small merchants without a bricks-and-mortar place, or anyone with limited space.
My first encounter with the mobile cash register was buying a cheese haul from Kathleen Cotter of The Bloomy Rind. She tallied it on her iPhone, then scanned my credit card on a tiny square plugged-in attachment. In fact, the technology is called Square.
At the Farmers Market outlet of Arnold Myint's growing dine-o-sphere, called AM@FM, the combination of Square plus iPad has freed up the counter space that would have gone to a register or even a desktop computerized register. Compact, elegant, and no more dead trees, either — if you want a receipt, they send it to your email address. Saves me having to clean out the handbag each week.