Here's one I've been wanting for a long time.  I think it would be great to pair my cell phone with my computer and have the computer screen lock each time my phone leaves Bluetooth range.  Seems like a great security feature given the prevalence of cell phones these days.

To the best of my knowledge there are no Mac applications to facilitate this right now.  Fee apps or otherwise.  I read about one possibility a while back that was a series of scripts.  But when I played with it, it just didn't work correctly.  Over at phoenixfreeze.com, it looks we might have a solution on the way.  Right now their product only supports Windows XP and Vista.  But according to the FAQ section, Mac support is planned.

Its not world class security, but it could keep your workstation locked down should you forget to lock it before you head out of the office to lunch.  Its not even out yet, for that matter but it will be a release worth watching for.

A couple of gotchas come to mind.  Some cell phones will only pair with 1 device at a time.  This could be a big problem if you sit at your desk with your cell paired with a headset that is also in range.  It would mean that the phone could only be paired with the headset or the computer but not both.  I believe most of the modern phones support multiple Bluetooth profiles now.  To my understanding that would allow the device to be paired with more than one devices at a time.  But it does seem limiting to only pair a cell phone to one devices at a time.

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Steve