One of our cats waits when my girlfriend and I are at the dinner table, then leans against the table with two legs, touches my GF arm with one of his paws and sometimes emits a meow incredibly similar to what in a local dialect translates to "hey!" which probably he already heard a thousand times from us. That's his way to tell us he wants some of our food, and he doesn't stop until he gets some. Not that it counts as a sign of intelligence, anyway I was able to teach the other cat to shake "hands".

To me the sign that cats are definitely intelligent is their ability to make us think so whether they are or not:).