I Heart Twitter
First, now it is possible to sign up without having a mobile phone. This allows you to "kick the tires" and try out Twitter without having to "commit" so to speak. The second feature, which is more insanely useful, is you can send/receive updates via any Jabber-enabled IM client sending to [email protected] You also receive the Twitter updates via IM as well.
The only problem I see with the IM/SMS feature right now is that if you are on via IM, then you won't get SMS notifications. I usually leave my IM client on all the time, so that wouldn't work too well for me. I'd like to see a bit more control over that, personally.
I still have to wonder how they are going to monetize this service, though.