Bruce Schneier linked to this Wired article about Radio Frequency ID (RFID) tags. It will fascinate or terrify you, depending on how thorough and disciplined you think the implementations will be. Choice passage:
"I was at a hotel that used smartcards, so I copied one and put the data into my computer," Grunwald says. "Then I used RFDump to upload the room key card data to the price chip on a box of cream cheese from the Future Store. And I opened my hotel room with the cream cheese!"