I use a program called Proximity -- http://www.apple.com/downloads/macosx/system_disk_utilities/proximity.html -- to do the following:
- When Proximity can't detect my Bluetooth phone (which means I'm not at the computer), it: turns on Adium's away message, pauses iTunes, turns on the screensaver password setting in System Preferences, and then activates the screensaver
- When Proximity detects my Bluetooth phone again (meaning I've returned), it: sets Adium back to available, turns on the screensaver (without requiring a password), and then turns off the screensaver password setting in the System Preferences