Eighth Commandment revisited

In the combox in the blog on the Eighth Commandment, Jane M. raises some interesting questions. (Actually, all the comments are worth reading, but I figured this on deserved some space, given the nature of blogs.)
Jane M. writes:

Here's one I find hard. If several of us witness the same really bad behavior by someone is it okay to talk about it amongst ourselves? (It is true and nobody is lowering anyone else's opinion because we all saw it. But it gives me a squeamish feeling along with the human pleasure of venting...) But then, what happens when another member of the group who didn't see the offense is present for the talking. I think there's something wrong but haven't nailed it down. Here's another. What about talking about someone you know your listener will never meet? I have a friend who just always needs to talk about the behavior of her in-laws and it isn't pretty. But I don't know them and she wants to vent. I think this commandment is broken a LOT.

What do you think?