where have you been living the past few centuries? ;-)
It's me! = correct as subject
It's him again! = correct as subject
It's her on the phone. = correct as subject
It's them. = correct as subject
I have NEVER in my life written "but not he" (in the subject case). "It is I" is acceptable biblical English, but I would never use it.
Please note I am not saying that "he/she" is incorrect, and I consider it a bit silly to insist that "him/her" in subject position is wrong. http://www.drgrammar.org/faqs/It's her or it's she?
Patricia O'Conner, author of Woe Is I, says, "It's OK to use It is me, It's her, and similar constructions, instead of the technically correct but stuffier It is I, That's he, and It's she.... Unless you're addressing the Supreme Court or the Philological Society, you can drop the formality
(186). [my emphasis]It is I or it is me?
It is I or it is me? According to the Merriam Webster's Dictionary of the English Language,"...instead of the old choice between right and wrong we are now choosing a style; it is a choice that is much closer to the reality of usage than the old one was...Clearly, both the it is I and it's me patterns are in reputable use and have been for a considerable time. It is I tends to be used in more formal or more stuffy situations; it's me predominates in real and fictional speech and in a more relaxed writing style. Him, her, us, and them may be less common after the verb to be than me is, but they are far from rare and are equally good
" (566, 568).
Patricia T. O'Conner in "Woe is I" writes that the practice of using the nominative case of the pronoun following a form of the verb "to be" is all but extinct. She says that all but the fussiest grammarians would accept "it was you and me." The American Heritage Book of English Usage online agrees that "It is me" sounds better and more natural than "It is I."