Ian, a co-worker and passionate Mac supporter, pointed me to this remarkable piece of software for OSX users with an Ericsson bluetooth phone.
Not only does it allow you to use your phone as a remote control (great for presentations) but it senses your proximity to the computer and triggers actions when you go into and out of range.
I tried it on my machine and it works perfectly. I used it to control iTunes from 15 feet away. It did take a few seconds to recognize when I was out of range and when I returned, but I assume that is some kind of latency with Bluetooth. I doubt it's a problem with the software.
Want more testimonials? MacSlash has picked up the story.