A scruffy-looking guy strolls through a supermarket, stuffing groceries into his baggy trench coat. A security guard tails him as he heads toward the exit, passing through what looks like a metal detector. "Excuse me, sir," snarls the guard, snatching a piece of paper from the machine. "You forgot your receipt." It's a tantalizing fantasy of how easy shopping could be in a cash-free world, as envisioned in a TV ad by IBM. But while it may sound far-fetched, we're not that far off.
On a recent Thursday in Tokyo, I wedge my way through a crowded Sankus convenience store, picking out...