Or broadcasting to the world? According to Bojo it turns out that due to the way that twitter's api works at the moment, all that carefull 'only tweeting to friends' or 'blocking' is completely circumvented by pulling a feed via the api. Does this matter? not sure myself- I never really saw the point of private tweeting, or at least, I can see the point, but it is essentially different to the main, core twitter proposition.
I mean, I have tried to follow people that I thought would be interesting, and found them blocking all and sundry, and felt a vague sense that they weren't really playng the same game as the rest of us, and similarly, I have only very rarely been tempted to block. In essence, I see twitter as fundamentally public- those are the rules, use twitter to publish stuff and we'll all play nice.
i can see a role for a more exclusive twitter type service too, one where it's more invite only. I think the distinction between the two approaches is only now becoming evident at all, as usage blossoms and critical mass is reached. If Ev or anyone else reads this, I'd like to suggest a bifurcation of the twitter service, into private flocks and public mobs, or whatever the correct zoological term is for describing homogenous and heterogenous groups of birdies respectively. Covey and flock would seem to fit the bill. (see what I did there?)