Origin: before 900; Middle English me, Old English mē (dative and accusative singular); cognate with Dutch mij, Old High German mir

Usage note 2. A traditional rule governing the case of personal pronouns after forms of the verb to be is that the nominative or subjective form ( I; she; he; we; they ) must be chosen. Some 400 years ago, owing to the feeling that the postverb position in a sentence is object rather than subject territory, me and other objective pronouns ( him; her; us; them ) began to replace the subjective forms after be, so that It is I became It is me.

-- http://dictionary.reference.com/browse/me