RFID is coming in a big way. If you aren't familiar with the term, RFID is a way to mark objects by applying a "tag" to them and identifing them using radio technology. The tags contain a unique identification number, and a reader can send them a signal that will cause them to transmit that id back to the reader. This technology is already used extensively by libraries (in each book), and as electronic access cards in many corporations.

RFID is growing in popularity quickly. Both Walmart and the Department of Defense have mandated that all of their vendors tag their merchandise with an RFID tag. Many other segments of society are moving towards using this technology.

The problem is that there are large security holes in various areas. If you have a card key to access your building at work, a stranger can stand beside you, and with special equipment can read your card without you knowing. They can then go to your office and play back your cards id to gain access to the building. New cars use similar technology instead of traditional keys, and similar problems exist.

Another problem is that some RFID tags can be reprogrammed with new IDs. On this website, Jonathan Westhues discusses cloning equipment built and techniques he's discovered.

Link