David Aaronovitch has just provided 'The 10 golden rules of Twitter'. The final one is the 'rule of rules':
'Never, ever tweet anything about anybody that you wouldn’t say to their face. There’s a REASON why you wouldn’t say it to their face. They might hit you, or sue you. So why would you want to tweet it?'
I think it's a problem with metaphor. When a new form of communication comes along, we see it in terms of an older form. Tweeting is part shouting at the TV, part nattering on the phone with a friend. But you can't get sued or arrested for shouting at the TV. (Well, not yet; let's wait and see on that one.)
This applies to email. The problem is in the 'mail' bit of the word. Some people see it as like writing a letter. I always try to think of it as a postcard. You know those offices where people pin up postcards people have sent from their holiday and any passer-by is allowed to read it. Emails are so easy to forward. I try not say anything in an email that would be a problem if it were printed out and stuck up on the wall.
(On the other hand, I'd rather people didn't print out my emails and stick them up on walls.)