I'm not sure about the chemistry involved, but I've tried it twice. Once in a bowl with no effect, and once in a joint with a definite effect. Since the bowl had a flame on it the whole time, I'm guessing this means a flame is hot enough to break down the Psilocin and Psilocybin, but a lit J is not.

I wouldn't do it again, though. We tried it those two times because we just had leftovers from a bag that didn't get eaten for some reason, but not enough to trip orally. The effects only lasted about 30 minutes and felt a bit like my head was going to explode. The only think about it that even reminded me of a mushroom trip was this weird curly kind of look everything got to it, which faded when that feeling in my head did. It was nothing spectacular in the least.