The verb "to be" is intransitive, it never takes an object, therefore the correct form is the nominative (I think that's what we called it in Latin). "It is I" is correct. "It is me" is incorrect, but it's what everybody says, and the world won't come to an end if we're all wrong. Or so I've been made to believe.
Next you're gonna tell me the London bombs were in the subway, right intrainsitive view of any fellow rider.
you are the smartest person i know, arghy : )