I can't really tell you if its a good idea or not, but I can tell you that there might be other ways of making your intent obvious without being so blatant. I've had success writing extensive thank you notes to schools I am interested in after an interview, starting with a thank you for the opportunity to interview, and following up with a list of reasons why the school so attractive. Complimenting the features of the program that are attractive is an easy way to show that you were impressed. A lot of people write thank you notes from what I understand, but being very detailed about WHY you are thankful is probably a good way of saying I AM INTERESTED without coming out and saying it. Also, when I write more extensive thank you notes, I tend to actually get responses back, and it is usually obvious from the response how the school felt the interview went.
In general, though, if you are 100% sure you want to go there, it never hurts to be honest. I heard a story from a program administrator who got an email EVERY SINGLE WEEK from a person on the wait list, but when it came time for the committee meeting to decide who would get a spot that had opened up, she knew exactly who to offer it to, because they knew the applicant would take the slot, even that late in the game. Obviously don't go crazy like that person did, but you get the point.