Corporate users have had it available for a while: presence management for verbal communications.
Now Jaiku is providing it for the rest of us, as long as we have a Nokia Series 60. I can't test it out as neither I nor the majority of my friends have a Nokia phone, but the application provides presence, location, and availability information for users in the phone book.
This is the sort of application that could provide stickiness for a carrier if it were given free to its users. Perhaps we'll see a version written in uiOne or MIDP2. It'll take a while.