Okay, seriously people, it's "duct tape," as in tape for taping ducts, not "duck tape," which I suppose would be used for taping ducks, assuming you ever wanted to tape a duck, which I don't know why you would. I don't imagine the duck would be all that thrilled about it either.
And yes, I do know that there's a brand of duct tape called Duck Tape, but if you want to refer to that specific brand name, then it needs to be capitalized as befits a proper name.
And yes, I do know that there's a brand of duct tape called Duck Tape, but if you want to refer to that specific brand name, then it needs to be capitalized as befits a proper name.