wsieber asks, "What's wrong with: That's the big hill Jack and Jill went up ?

A good question. Which gets us to the simple fact that the rules, such as they are, are reinvented every generation. We learn what sounds right by the time we are about three and anything that doesn't sound right is wrong, regardless of what some defender of the received rules may say. If this weren't the case we would still be speaking an inflected language in which relations between words would be expressed more by case structure than by word order and we would know the difference between prepositions and phrasal verb affixes. As it stands we have nothing to rely on except William Safire's rule, "If it sounds funny, the hell with it." Most of us who think that "it is I"* sounds funny will continue to say "it's me" and let the linguists worry about whether the verb to be has gained a transitive sense.

*Does anyone argue that it should be "it am I"?