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I use this to lock and unlock my Mac, at work, based on the bluetooth connection between my phone and computer. Once my Mac detects that the phone is unavailable it locks itself.

What you’ll need:

• Proximity 1.0 a free download from Apple.
• A compiled version of notify.c
  #include
int main(int argc, char ** argv)
{
CFMessagePortRef port = CFMessagePortCreateRemote(NULL, CFSTR("com.apple.loginwindow.notify"));
CFMessagePortSendRequest(port, 500, 0, 0, 0, 0, 0);
CFRelease(port);
return 0;
}
  You can download a pre-compiled version here, or compile it yourself (Apple’s Dev Tools required) with the command
  cc -arch ppc -arch i386 -o notif notif.c -framework CoreFoundation
• An Apple Script to set the screen saver
  -- Turn on the screen saver password
do shell script "defaults -currentHost write com.apple.screensaver askForPassword -int 1"
set MyName to do shell script "whoami"
set runFile to "/Users/" & MyName & "/notif"
-- display dialog runFile
do shell script runFile
-- Activate the screen saver
tell application "ScreenSaverEngine" to activate
  N.B. You’ll need to change “/notif” to match the location if the compiled notif binary
• An Applescript to unset the screensaver.
  -- Disable the screen Saver Password
set MyName to do shell script "whoami"
set runFile to "/Users/" & MyName & "/notif"
do shell script "defaults -currentHost write com.apple.screensaver askForPassword -int 0"
do shell script runFile
-- Turn OFF the screen saver
tell application "ScreenSaverEngine" to quit
  N.B. You’ll need to change “/notif” to match the location if the compiled notif binary
• Connect your bluetooth device to the Mac via bluetooth.
• Run Proximity, set your device, set the Applescripts

Useful Links:

• Lock Computer Using DeskShade
• Bluetooth Proximity Detection on OS X
• Automatically turn on/off Password Protection for the Screensaver

Update:
Mac OS X Hints has a much better setup.