Jesse David Hollington put together a set of scripts that automatically disables and enables his screensaver password when his cellphone comes in and out of Bluetooth range.

Ideally, I would want to activate the OS X screen saver and enable the password protection when I move away from my computer (out of Bluetooth range), but otherwise I’d prefer to keep the screen saver password off for normal use, as it gets quite annoying when I’m working near the computer to have to continually re-enter my password after I’ve diverted my attention elsewhere for a few minutes (which happens frequently, as often the Powerbook sits to one side of other systems that I’m working with, rather than being in constant use).

With a few Applescript apps and some quick C code, his computer will automatically unlock for him when he comes near it. –Link. (source included!)