According to Laurie Bauer, professor of theoretical and descriptive linguistics at Victoria University of Wellington, New Zealand, the "It is I" rule is just one example of how the rules of Latin grammar have been inappropriately forced on English.
In the 18th century, Latin was widely viewed as the language of refinement--classy and conveniently dead. As a result, a number of grammar mavens set out to transfer this prestige to English by importing and imposing various Latin grammatical rules--regardless of actual English usage and normal word patterns. One of these inappropriate rules was an insistence on using the nominative "I" after a form of the verb "to be."
Bauer argues that there's no point in avoiding normal English speech patterns--in this case, "me," not "I," after the verb. And there's no sense in imposing "the patterns of one language on another." Doing so, he says, "is like trying to make people play tennis with a golf club."
Amazon.com: Language Myths (9780140260236): Laurie Bauer, Peter Trudgill: Books