I'd like to comment upon the status of the Nominative in English, particularly the predicate nominative. Usually, in English, the Nominative and Accusative (direct object) cases are identical, except in certain situations that are very imoportant, namely situations involving pronouns. Me, you, him, her, it, us, (y'all), and them are the accusative forms of I, you, he, she, it, we, (y'all), and they. As far as I know, these are the only accustaive forms that appear on the surface in English grammar (Accusative case in English is normally dictated by word order, not inflection, especially now that are verbs are for the most part uninflected), except for of course, the reflexive forms myself, yourself, himself, herself, itself, ourselves, yourselves, themselves, which are different monsters altogether.
Now, in most European languages, the object of a verb of being, such as 'is' (even when it's not stated, like in Russian ja amerikanets -- I (am) (an) American), must always be in the nominative, since this is in essence a statement of equality, not having anything to do with one thing acting upon another, which would yield an object. So, in the sentence 'John is a jerk,' both of the nouns are in the nominative case, as would be any adjective describing jerk. and in languages with case inflection, such as Czech, the subject and the 'object' of the being verb are both in the nominative : Ludmila je dobrá studentka 'Ludmila is a good student', not * Ludmila je dobrou studentku. This sounds very wrong.
Now in most languages, pronouns function much like the nouns they are filling in for, and in most languages, the predicate nominative presents no obstacle to this. In Spanish, you'd sound like an idiot if you said *es mí, soy mí Or let's look at a language closer to home, such as German where a dummy subject 'it' is used at the beginning of a sentence: Es ist er, natürlich 'It is he, of course.' Now, look at how silly the English translation will look to most of you. What's going on? To a presrciptivist, of course, this is perfectly correct in every way. But does anyone else rememebr sitting in elementary school saying 'what the hell?' to this? I do.
In Modern English, it seems that it sounds more natural, perhaps in some cases more correct, to use the accusative form of the pronoun after the being verb. When you're knocking on your friend's door and they yell 'who is it?' 'It's me' sounds much more natural than 'It is I.' 'Who's been eating my turnips?' 'It was she!' They always tell you that you'd have to answer the question 'she has been,' so then, logically, 'It was her!' is wrong. This is also the reason why it is technically wrong to answer 'Me!' to a question such as 'Who won?' But as the folly of the no double negative bullshit rule has proven, prescriptivist 'logic' and syntax can often butt heads. Most people, at least in the States, who use forms like 'It is I' and 'It was she' sound like a.) bourgeois snobs -- the socially mobile (or so they think) 'new-money' people who constantly prove that you can have a lot of money but you'll never be upper class no matter how much you want it (this is where Chevrolet and Hummer get their entire market for Avalanches and H2s). I can almost guarantee you that the 'correct' form appears more often in upper-middle class speech than in actual upper class speech (See William Labov's 'fourth floor' study in New York for this phenomenon in action).
Or, you could sound like b.) a non-native speaker of English who has learned from a textbook which declares these forms as correct.
In both cases, your speech will be highly marked, either as a snob or a foreigner. I don't mind foreigners, and am in fact intrigued by systematic mistakes of non-native speakers, but I can't stomach snobs, especially the ones who habve nothing to be snobby about. I remember being with my girlfriend and her family looking for the presentations of some architectural projects. when we found them, my girlfriend said 'Here they are,' to which her mother responsed 'Oh, these are they?' I wanted to laugh out loud at this and how what she thought would make her sound smart only made her look like a twit. She's from Joliet, Illinois, and I can bet anyone reading this that that form is NOT in her native grammar. Any speaker of any standard American English dialect would immediately find that construction as odd. But, in all fairness, any alternative is awkward: 'This is them, these are them,' all sound kind of funny, but none as bad as 'these are they.'
So the question to a theorhetician is 'are these accusative surface forms actually accusative?, and if not then why do they show up?' Maybe it's because speakers are putting objects behind all verbs, regardless of its grammatical function. This is definitely hardwired into the grammar of the speakers who use these forms. But does that actaully make them accusative? And what implications does this have for the nouns they are replacing? If we construct 'It is him' as S-V-O ( as opposed to S-V-S), then what doe sthis say about 'It is John.' Is the underlying cas eof this S-V-S or really S-V-O. I don't know the answers to these questions. All I know is that my girlfriend's mother cringes when I say 'It was me and him' and even 'him and her are getting married' (this is highly nonstandard, and has nothing to do with the predicate nominative, I admit). But what do I care? I'm a linguist, and my experience as one has made me proud of this. Samuel Johnson can shove it up his split infinitive.