Certain models of cells phones by Docomo and AU in Japan act both as your phone and you're credit / debit card. The technology is driven by contactless IC cards.

Vending machines, ticket wickets and even some bullet trains now have the IC proximity sensors to detect a nearby phone. This technology is espeically prevelant in train stations throughout the Tokyo area. With the ticket wickets, you just pass your phone over the sensor and you are charged, no confirmation or PIN required. According to the Docomo site, you can either opt in for a debit style account or a credit card account. There is a disclaimer that if the amount being charged exceeds a certain level a PIN will be required. Docomo is calling these new fangled cell phones: cell phone wallets.

So to answer the question above, they can't really. Just hang on to that cell phone real, real tight.