There is also software you can use that monitors for Bluetooth devices and can perform actions based on them. Makes it very easy to roll your own piece of software and not have to trust a 3rd party device: http://www.macupdate.com/app/mac/23858/proximity

I am not entirely sure why you would need a dongle over a phone. Perhaps the the creator can chime in here and explain to us why we would chose a hardware device over what we already have in our pocket?

If we get an answer to your question, it will be forwarded here.

Great idea, but I don't want another fob. If it could be made to work with a phone then this would be perfect.

http://www.knocktounlock.com/ comes to mind.

Edit: Sorry, it appears Knock can't actually lock your Mac. Airlock promises that functionality though - so that might be of interest to you.

Thanks for contributing with that link :)

To be honest, i don't know the answer to that question either. But I'm guessing there's a good reason why they decided to provide the solution in this form.