My dad's known for a while, he doesn't really agree with it in terms of how "safe" it is but if I think it works he's not going to contradict me unless he sees something happen that he feels like he needs to warn me about(bad thing is a lot of the time he'll attribute some problems I have to being multiple when it had nothing to do with the situation).
A few friends know but not in a big "comming out" way. It was just offhandedly talked about(same way they found out about me being a pagan, go figure) and they haven't really questioned it at all and took it in a "that's how it is way" instead of a "what's wrong, we need to fix it way". And my boyfriend knows....but he's kinda multiple too so that works out well. I don't think I have a good picture of how it is to be seen as "different" or something. I basically live in a very un-biblebelt niche of the biblebelt.
I wouldn't dare tell my mom....she was threatening to send me to a psychward for being "uncooperative" meaning saying I wasn't depressed when she told me I was depressed.

I think the key is to make sure you have a pretty good relationship with the person in question, which may mean telling one parent and not the other or just not telling them at all.