It's the risingest next thing - growing faster than Facebook, which is growing faster than Myspace, which is growing faster than blogging, which is growing faster than ... well, you get the idea.
I'm not convinced, personally, and I'm certainly not interested at least for now. 140characters - the length of a SMS message, less than the maximum length of a Facebook status - is too restrictive. I want exposition, expression, images .... You can do amazing things with the internet - so why constrain yourself to using no more than 140 text characters? That's not to say that you need to do more than that - and the social networking status messages are pretty similar to Twitter. But to say that you can't do more than that?
As for the fact that celebrities are doing it - well, to be honest, that's a very good reason for staying away. I'm not interested in what famous people say when they get stuck in lifts, or even that they have been stuck in lifts. So what? It happens! I would be interested in what my friends were thinking and feeling, if they get stuck in a lift - but not simply the bare fact that they are stuck.
Expecting flames ....