English as she is spoke.

I was out doing my usual morning walk yesterday and I invariably pass and say "Hello" to a particular lady at a particular point, but yesterday she wasn't there. And then I saw this figure approaching in the distance and thought to myself "that's her", and then - weird this - I clearly heard the voice of my old English teacher inside my head saying "...that is she, Random, that is she!". But has this rule that the verb "to be" must be followed by the nominative any real validity today? It seems to have taken root in the 17th or 18th centuries, and to be based on the idea that as the Romance languages are all derivatives of classical Latin, they should follow the grammatical rules of that language, and certainly in Latin there is such a rule. But as I've said before more than once, language is essentially work in progress and - certainly in speech - is constantly developing. I would guess that 90% or more of people would automatically say "it's me" rather than "it is I". So as is often the case, we have grammatical correctness versus common usage - and you take your pick.