In English as well as in German the correct case is Nominativ and its respective counterpart in English. It is true that colloquially one might use "It is me" and in fact a vast majority use it that way. If you are studying in Oxford or Cambridge, though, you will be trained to answer "Who is this" by "It is I".
The same with independent clauses, i.e.:
It is I who have learned english. correct
It is me who have learned english. incorrect
In this case the latter is not even colloquially possible but outright wrong.
In german this distinction between colloquial use of the object case and grammatically correct nominative doesn't exist, always Nominativ is used:
Ich war das.
Compare with http://german.stackexchange.com/questions/7631. (Oops, was did not see that it had already been mentioned.)
– Carsten S Sep 09 '13 at 08:28