Yes, I have just got to say something here. If you remember, a long time ago, that was a request I had. I am really glad to see it finally being implemented. Back then, we were talking about hiding the tree, etc. We never really came to an agreement on it. But yes, by all means, hide the username, when the MPW is not supplied.
As I said back then, the username is one piece of the puzzle, which any hacker trying to break into your account will need to solve. Why give them half of the answer up front?
And I agree with tanstaafl. Make it hide and do not give the user a choice. But then, I see your point, too. If you give the user a choice, you should make it checked by default, IMHO. - Just look at my tagline.