one thing that came out of a discussion with a friend of mine just now: a cool thing that old twitter (pre-2007) had going for it is that you didn't have notifications. this sounds like a bad thing but it's actually good, because you didn't have spam either. you only saw what you explicitly followed.
mostly what i conclude from this is that it reinforces the value of having broadcast and notification being disjoint mechanisms. you should be able to broadcast without needing an inbox.