I asked my OB GYN abou this last year- he was unaware that it was used by body builders for the purpose of avoiding estrogen-related fat. He did say that it is used for breast cancer patients, but most of the research around it is in post-menopausal women. I was using birth control at the time and he did say that it would effectively "neutralize" the b/c so you don't get the intended protection there. He said that there wasn't much known about use of nolva in pre-menopausal women. He suggested I just go off my b/c. I never ended up using it.
Not much, but this is what I found out at least from the doc.
Good question for thebabydoc!