I'm also very intrigued by this topic. All my close friends know and most are pretty accepting of it, the main thing was being able to clarify and answer questions they had. Be ready to explain your definition of "open" or "poly" and be ready for lots of "advice" about if its a good idea or not. Once they understand the why they are more comfortable with it. I have told a few acquaintances from my job, but never someone above me, like a boss or something. Many family members know and accept it, even if they think its odd.
The person I could never come out to would be my mom, she would never understand. Thankfully, the likelihood of her finding out is extremely small so I'm not worried about it. However, I do worry about J's mom finding out and I'm wondering when we will have to face that situation.
In general, I feel pretty okay with telling people....friends, acquaintances, my family members (excluding mom). I'm more nervous about J's family members, I'll let him decide when its a good idea to reveal it.
The main coming out problem I'm having right now though is a situation where even though its my reality I don't feel comfortable because it could put someone else in an awkward position. I'll explain. I have a friend who I want a relationship with, we've talked about it, he knows I'm poly, but he decided he didn't want to be in a relationship with me. I respect his choice and when we hang out I try to stay in the friendzone and avoid being flirty or touchy, I have rules I set in place to keep me from disrespecting his decision. He, however, does not. He is very physical with me and his roommates definitely notice. Anytime he is more physical with me than he "should be" his roommates give us looks like "um whats going on here??". I don't feel comfortable telling his roommates about the situation because I'm revealing my friends part in it. But at the same time, they probably think he is hitting on a married woman and are confused by it, and even though I don't usually initiate the intimacy I definitely don't discourage it so they also probably think I'm being unfaithful to J.