We've seen what happens when you Twitter everything , and it's not attractive. But we all have different levels of what we're comfortable sharing, and some people are ready to (over) share a lot more than others. Erykah Badu, for instance, just had a baby. No, I didn't see a press release; she Twittered it .
While I follow some of my favorite Twitterers, like Dooce and Tina Fey  (that's one of her more tame ones pictured there), simply because they have a habit of excellent TMI, I myself would never say just anything on Twitter. I keep it light, simple, and try not to turn anyone off their lunch.
And particularly since Twitter is big on accruing a ton of friends/followers that you often don't know personally, it's even more likely that I'll keep the personal stuff to myself.
Not that I'm anybody, but still, I wouldn't share anything I wouldn't want my mother to know. That's my rule — what's your rule for sharing stuff on Twitter or other places online?