So now that you are part of the "VIP" crowd and you are one of those special friends that is allowed to see your friend's private tweets, let me ask you. Is it acceptable for you to ReTweet something that is has been marked as private? After all it was private to start with but at the same time it could be some nice piece of information that you want to share with the word and you want to give credit to your friend.
It is as easy as writing "RE" in front of your ReTweet you are passing along or putting a (via @fancyNameGoesHere) at the end of it and it is again out in the open and the "privacy" is out of the window.
So I say, think accordingly before you ReTweet those private Tweets.