In certain situations, I love to talk about being a vegan and will happily discuss it. I'm really careful about how/when I do that, though, because I want it to be as effective and positive as possible. I don't specifically avoid mentioning it--i.e., if I'm going out to dinner with someone and they mention going to a steakhouse, I would have no problem saying, "Actually, I'm vegetarian"--but I don't always volunteer it in every situation.
For instance, last week I was picking up my boyfriend after a seminar at his aikido dojo. Several people were eating lasagna, and they kept offering it to us. At first we just said, "No, thanks," but after they kept insisting, we just said, "Actually, we don't eat dairy." They were sort of like, "Oh, you're THOSE kind of people." I don't think it was a bad thing, but I don't think it had any positive effect, either. Whereas in certain other situations, talking about it can have a REALLY positive effect, and get people thinking. Personally, I think of it as my job as a vegetarian/vegan to figure out the difference between these situations, and just be as positive an influence as I can be whenever it's possible to do so.