Yes, most people think it has to do with weight, some say it has to do with the circumference of the coin. I've also heard the conductance of the coin is measured.
Anyways, you are definatly on the right track with the frequency, I'm thinking you arn't going to see many if any counterfit coins unless you are building this for a country that has strayed away from paper money? Anyways, if we are talking small values for each coin, I dont think anyone is going to take the time and effort to figure out at which frequency a coin oscillates, and try to replicate it.
Givin that, I'm thinking that any extra circuitry used to detect a counterfit coin verse a valid coin, would cost more them if someone actually did put in a few counterfit coins...
So... all you really need, is a surefire way to detect the value of the coin that is inserted.