I'm fairly open about it. Enough that people that I aren't really aquainted with would possibly know. During the school year which recently ended. I was in a position where I was known to most of the student body (Well, most of the senior student body), being a Prefect. And I've friends who openly do pony related things on their Facebook and discuss it in public. While I join in. I wouldn't initiate.
For example, last week I learnt that one of my friends knew because she texted me that there were MLP cookies on sale at the supermarket. I had no knowledge of her knowing before that fact.
That being said, I dislike people who would balantly try to push bronyism, for lack of a better word, onto others relentlessly. I've converted roughly 7 people by now, and my 2 closest friends are adamant about MLP. I've had them watch the pilot episodes and a few others, they laughed, yes, but said they wouldn't be interested in the rest of the series. So I dropped it.
Anyway, people don't sem to mind. Well, people I'm aquainted all the way to close friends don't. There's sometimes some light joking, of course, but that's true of all things, really. Especially with the New Zealander culture.
My two bits.