This is just a concept, mind you, but its a good one. In response to an industrial design contest at IDFuel, Nathan Lynch and Lea Miller had proposed a Bluetooth enabled luggage tag that lights up when the cell phone its paired with is nearby and sends the phone a message. The tag also contains contact information for the owner of the luggage. Essentially, this is presence for physical objects and there are all sorts of applications that spring to mind.
Bluetooth Enabled Luggage Tags by Phil Windley: