Well, if you wanted to carry it to this extreme, even for fun, we could say that what we refer to as "unconscious" people, whether they are in a coma or asleep, are probably not all that "unconscious". Brain activity suggests that they are probably dreaming in some sense of the word and thus still "exist" even if they appear immobile. This also suggests that there's probably some truth to the idea that talking to people in comas may be effective in some sense or another.
As for the newborn baby thing, that brings up the whole question of when a baby arises to consciousness and assuming that there's very few anti-abortionists around here we should all have considered, at least to some degree, this ethical question otherwise it would logically follow that two slices of bread and a knob of butter might prove an effective family planning tool...