This is an excellent howto about setting up a bluetooth proximity system.
Have you ever wished your computer would keep track of your comings and goings and secure itself accordingly (when you walk away from your computer, you want the screen to lock. When you come back you want the screen to unlock.)? Odds are good that if you have a newer laptop and cell phone you can do just that
Many laptops now come with Bluetooth chips and most phones now support Bluetooth as well. This can allow your computer and cell phone to "talk" and create a simple proximity badge for you to use. Dan Elder explains how to set up the connection between the two and then have your laptop lock itself when you're away.