Not long ago, I bumped heads with a poster on another site over the use of "It is I" versus "It is me."
I felt secure in my support for the former usage and defended it mightily. I subsequently found some equivocation on the part of several web grammarians who, while acknowledging the grammatical correctness of "It is I," added words to the effect that "It is me" has become acceptable, at least in non-formal writing. I have to admit, I felt a bit let down. At the same time, I would never respond to someone's question, "Who's there?" with other than, "It's me."