Well there is something to be said about how much people can pick up without realizing it. Because of this, I think it's possible that if the person administering a placebo knew it was a placebo, the person receiving it could pick up on that somehow.
As long as the doctor that is administering the dose is aware that it could be a placebo then I don't think there is any ethical dilemma as placebos are usually only used in studies. (Unless you're Doctor House from the show House M.D.)