When I was in NYC a while back, I met a woman at a friend's dinner party. She sounded totally American, but was in fact Brazilian. She worked as a lawyer, and said that she'd had to get extensive voice training in order to sound American so that people would take her more seriously professionally. I have no idea if the professional part worked, but the accent, mannerisms etc was amazing - I would never have guessed.

I'm referring to speaking, not singing. After a _lot_ of work, I can speak passably as a woman or man and switch freely between the two. Depending on context I generally choose just one for the entire conversation, as switching tends to cause whiplash in the listener (^_^).