Lots of people have thought of writing "twitter but open". Congrats on actually doing it. When I was kicking around the idea but never acting on it, one thing that occurred to me was that, in order to make it really take off, you have to create some way that everybody already has it. Some way to follow and be followed by twitter users, for example, or a way to make it so that your e-mail address is your microblog address. Just my two cents.