Many states have made it a crime for a person who knows he is HIV positive to have unprotected sex with another person without disclosing HIV status, having done so to clear up several issues (mens rea, causation, etc.) that would arise in a plain old murder prosecution. These crimes usually carry sentences similar to manslaughter, which is 8-25 years in typical states. Not all states have done this, and prosecutions are rare. I recall one in Tennessee that involved a female prostitute who had unprotected sex with upwards of 25 johns, infecting an unknown number of them; I don't know how the case was resolved. My inclination is to support making knowing infection equivalent to murder, but I see the arguments for treating it differently.

"Ethical obligations" to me seem indistinguishable from moral obligations, which the previous poster has already opined on. I consider it an ethical obligation to disclose status if you know you are positive, even if you think (but don't know) your partner is positive or knows you are.