dummy 'it'

[Matthew Gordon]

Can someone give me the standard line and/or references on the analysis of
"it" in sentences such as:

"Who is it?" in response to a knock on the door
"What is it?" with the meaning of "what's wrong?"

Labov in his classic article on copula deletion and contraction refers to
this as "dummy it" (and of course notes that contraction/deletion is not
permitted in this context). Is this "it" syntactically the same as the dummy
subject in, e.g., "it's raining"? Obviously it's not the same in that
contraction is permitted in the latter case, but I guess I'm asking if
syntactians would label both of these as the same "dummy it". They don't
feel the same to me, but then I'm not a syntactician so I don't put so much
faith in my intuitions.

-Matt Gordon