I would give this a half and half, but since I cannot do this, I must vote affirmative. I personally believe the deciding factor is how you do it. Because what I believe is based on a case by case study, I won't describe it here, but a key aspect of it is if you do something else unethical to download it, then the downloading becomes unethical. Similar to how in most states, if you have a concealed weapon permit but you are caught committing a felony, the permit becomes void and carrying the gun is illegal.
It has not yet happened to me, but it is not like you are unwilling to pay for the disc. You have already participated in the industry by buying the disc, but you lost it. You are simply getting another one.
It's not cheating, just helping yourself. I don't think it's piracy necessarily, as that implies you have not paid for the product, when in fact, you have.