I might suggest that a useful assumption is that anything "important" will be shared. For one thing, it's really unpleasant to keep significant secrets from a lover or spouse. For another thing, it's only fair to try to keep everyone in the relationships on the same page. For a third thing, there's a sense of easiness that comes from being pretty sure that everyone knows the important stuff. It's good to avoid those "um...didn't she tell you...?" moments when someone feels the rug suddenly slip out from under his feet.
But for me there are other things that are private. Details of lovemaking, for instance; descriptions of smoochy moments; talk about the other person's physique. (Some couples find this exciting, so take that part with a grain of salt.) There is information that can be painful, and that should be presented gently and with consideration, and perhaps as it becomes necessary. That's something I have a great deal of trouble with -- I tend to blurt out the unvarnished truth.
As far as communicating with your wife's BF, I would tend to think that the general rules are the same. I remember that my GF's husband sent me a discreet email telling me when her birthday was, which was sweet. But again, I would assume that if you mention something that's really important to him in a message then he may mention it to her as well. It would be a little unfair to expect him to guess what to say and what to not say unless you specifically ask him NOT to mention something. And in that case the two of you are keeping a mutual secret from the third partner in the relationships, and if the secret is an important one I'm not sure how that plays in poly.
If you can look all the partners in the eye and say "Well, I've told you everything I think is important, and if you have any questions I'll try to answer them" then you're well ahead of the game. Etiquette, the nuances of doing it without hurting feelings or starting arguments, is really situational. Takes thought and care. Good luck!
Last edited by EugenePoet; 07-23-2010 at 05:33 PM.