Cool thread.
We live in a Spanish speaking environment. South America. In our house, and everywhere for that matter, I speak 100% English to our children. Been doing it with both since they were in the womb. My wife speaks Spanish, but helps out with the English from time to time when I am not around. But not much because she is afraid of making a mistake and teaching them wrong. In our house we do only TV and movies for the kids in English. All books we read together are English. The kids, currently 3 and 2, know with whom they can speak English (Dad, and Mom a little) and Spanish (everyone else, Grandma, aunts, cousins, etc.).
My way of thinking is they have a million opportunities to learn Spanish all around them. But only I can teach them English. So I do, and I insist on the most perfect, proper, grammatically correct, accent correct American English from them. Just like my parents did from me. It is fun, fun, fun.
I read somewhere the question, "How many languages can a child learn?" And the answer was, "How many are you willing to teach him?"
It is so gratifying to have a conversation with my 3-year-old that I could not have with the 20-somethings that are trying to learn English at the local English language school, because my 3-year-old can and they cannot.