grammar - What does "it" refer to in "it's raining"?

I wanted to leave the question title as is so as not to take away from my amusement :).

Anyway,

It's raining.

What is raining? Is it the sky? The clouds? The weather? The rain? What is "it"? Any historical insights on the statement?

Answer

Definitions for it in my Webster's 3rd New International Dictionary include:

2a used as an expletive subject of an
impersonal verb that expresses a
simple condition or an action without
implied reference to an agent about
the weather ... or time.

It is raining or It is two o'clock are examples of 2a.

2b used as an expletive subject in other statements or questions having
an undefined subject

If it hadn't been for your help, I couldn't have finished in time is an example of 2b.

3a used as an anticipatory subject of a verb whose logical subject is
another word or phrase or clause

It is I who have the answer to the question is an example of 3a.