Instant messengers with bluetooth detection facilities. That’s where it’s at, man.
If you’re on Linux, you’re probably a step ahead of the game already, given this page on the Gentoo Wiki (courtesy of Jim’s del.icio.us links) which shows you how to perform an action when your bluetooth phone comes into or leaves the proximity of your computer.
(I guess “Next-gen IM” is over-egging it a bit, but I do think it’s next-gen presence. Once Psi supports Ad-Hoc commands, I’ll be able to fulfill my dreams of changing my Jabber status based on my calendar too.)