personal pronouns - Can I use "it" to refer to a person?

I can think of two instances in which English speakers frequently (and perhaps normally) refer to a person as it.

Situation 1:

[Person A knocks at person B's door.]

Person B: Who is it?

Person A: It's Vito.

Here, what Person B wants to know is "Who are you?" and what person A is answering is "I'm Vito." But standard practice is to frame Person B, at this stage of the interaction, as it, not you. Perhaps this framework dates back to the days when the question "Who is it?" was directed to the doorman and not to the person doing the knocking (in which case the questioner would not know the sex of the visitor when addressing the doorman), but even then the knocker is initially labeled "it," not "our visitor" or "the person that knocked."

Situation 2:

[Person A sees person B carrying an infant.]

Person A: Oh, how cute! Is it a boy or a girl?

Person B: It's a girl!

Here the speaker's initial recourse to it is perhaps a socially cautious way to avoid misguessing the infant's sex, when that distinction isn't obvious from external appearances.

In both situation 1 and situation 2, the speakers are in no doubt that the person referred to as it is a human being; but for different reasons, it is an entirely acceptable way to style that person at that moment.