Wow, little did I know when I first started reading these comments, that they would still be going FIVE years since the very first...!

Okay, to me?
Grammatically, there can only be one absolutely correct way of phrasing it, and that would be "This is she". Grammar is a set of rules, admittedly some of it might be archaic and sound awkward due to linguistic drift, but the rules are pretty absolute. Whether you agree with the rule or not is a linguistic issue, not a grammatical one.

Language, however, now that is a whole 'nother kettle of fish. Linguistically, either is 'correct', or rather, nothing is incorrect, as language does indeed evolve.

Or at least, that is how I choose to understand this "debate". Oh, and if I had to choose, I would say "This is he", although more often than not, I preempt the question by answering the phone simply with "Hello, Jonathan."