In the 1972 movie, Play It Again Sam, Tony Roberts plays a business man who is constantly calling into his answering service (this is 1972 folks, long, long before mobile phones and such...) and reporting his whereabouts and leaving numbers where he can be reached. It is a running gag in the movie and is really pretty funny. Twitter is the web 2.0 answer to this.
"Twitter is a community based website where users post what they are doing at any given moment. Tell friends, colleagues or strangers what you are snacking on, your current interests, favorite websites or just plain goofy nonsense!"
You can update your Twitter information via a web page, a mobile phone, an instant messenger client and if you are running Mac OS X, you can update via Twitterrific, an application that lets you both read and publish posts or "tweets" to the Twitter community website.
Think of this as taking your instant messenger status (Away, busy, etc...) notice to extremes. :-) While I can't see doing this all the time, I can see where at conferences and such it might be fun to play around with. You can choose to make your updates public or private. I've added the Twitter widget to my sidebar so you can see an example of how this might work.