This seems like a rather unreliable way of protecting information on a laptop. I have a vague recollection of a program that uses Bluetooth to link a cell phone to a laptop. If the cell phone gets too far away from the laptop (if you walk away from your laptop, or someone runs away with your laptop), the program locks the laptop, requiring the password to unlock it.
Bluetooth is a decent way of doing it too. However merely locking the laptop is insufficient for protecting the data on the hard drive, since even when the screen is locked, the encryption key of the hard drive is still in memory. The solution is to securely wipe memory and shut off.