Have configured my somewhat ancient-but-still-serviceable Nokia 6230 mobile phone to echo my status via BuddySpace, kick in the screen saver and start or stop the music playing via iTunes on my iMac. Here's how:

First, pair your phone with your Mac using the OS X-included Bluetooth utlitity. Then, using the AppleScript editor, create two scripts along the lines of:

PhoneAway

tell application "iChat"
  set status to away
end tell
tell application "iTunes"
  stop
end tell
tell application "ScreenSaverEngine" to activate

PhoneHere

tell application "iChat"
  set status to available
end tell
tell application "iTunes"
  play
end tell

...and save them in, say /Applications/AppleScript
If you find that the editor complains that it can't find ScreenSaverEngine, it lives in:
/System/Library/Frameworks/ScreenSaver.framework/Resources

Next, download and install Apple's free application Proximity and point it at these two scripts. Pick your phone from the paired devices list and set the polling time to something like every five minutes.

Now when your phone leaves the office, your music will stop, your screensaver will kick in and iChat will let people know they can come and steal your things...
Technorati tags: bluetooth presence proximity detection screensaver iChat iTunes