Hi,
  forgetting/leaving behind your phone or other worthwhile items is very undesirable and can lead to all sorts of problems (such as someone breaking into your car for it). Hence the idea for the invisible teather. Many devices are bluetooth enabled or battery powered and could be otherwise tagged with an active sensor of some kind.

  A simple register and track device in your pocket or attached to the clothes or bags could keep polling the surroundings to identify if a specific gadget is responding. If it is, the device goes dormant again. If not, the device would alert the wearer that a specific gadget does not respond. The wearer can then confirm that the gadget is still in his/her possession or backtrack to check where the device was last.

  Another use would be to connect a gadget with a theft alert where the gadget starts making a noise once it detects the absence of the polling device. Hence will draw attention to itself. Finally, to find the mobile phone when it slipped into the couch cushions again a 'find-me' alert could be requested to track down (i.e. tuck the theather) the device.

Update 01/02/2010: Yes! Someone has already worked on this and apparently apparently another product has claims to be the first product based on bluetooth. Now, what would it take to extend this device to your bag, purse or wallet ?