Twitter is a kind of micro blog, a cross between a blogging tool and IM (Instant Message). Twitter is intended to let everyone know what you are doing right now. Depending on your security prefrence you may allow anyone or only friends to subscribe to your updates. In public most you can syndicate your entries.
Neptune now has an account on twitter (twitter.com) as you can see on the side bar here at neptune.dilated.org.
I see a log of potential in Twitter, espically the 2-way SMS allows you to update as well as keep keep track of your friends.