Research associated with the ‘Longman Grammar of Spoken and Written English’ shows that when the first person pronoun is final, me is more common than I ‘(It’s me’), but that when there’s a following relative clause, the opposite is true (‘It’s I who . . .’). That would suggest that It is I who am at fault is preferred over It is me who is at fault, assuming we reject the hybrid It is I who is at fault. Applying the same logic to the second person pronoun, we may suppose that It is you who are at fault is more popular than It is you who is at fault.