Because I'm an August MCATer and haven't received any interviews yet, I haven't decided what I'll do. I do think that thank you notes are not necessary and may not be as appreciated as you might think. I have been interviewed tons of times in a professional setting, and I have even conducted interviews. When I was interviewing for jobs in the legal market, I heard people tell me that writing a thank you note cannot help you it can only hurt you. If you feel like doing it, by all means do. But if there is a mispelling or gramatical error, or any other problem it can hurt you. If you send a thank you note, make sure there are no errors in it, and that you send it the day of or the day after the interview. If it's been more than a week, don't bother. It only looks like you didn't get around to it because you aren't really all that interested. I've never seen a thank you note help someone, I have seen it hurt people, and I've never seen anyone harmed by not sending it. I can't imagine it would be any different for medical school, where the interviewers are actually interviewing a lot more people and are even more inundated with thank you notes.
Having said that I usually send them because I do think it is polite. But if you don't want to don't worry about it. I received a number of job offers from prestigous law firms where I did not write a thank you note. And as for the original poster, I would advise not sending a thank you note two weeks after the interview.
I will probably send thank you notes to my top choices (if I even get interviews at those schools) and will not bother for the rest unless I really connect with someone or feel they were unusually helpful.