After speaking with a few MBA2-s, who are involved in the admissions process, the resounding opinion is that thank you notes don't really matter that much at all (most of them would even prefer to not get them). If someone has decided to say no, they'll say no regardless and vice versa. For instance, MBA2-s at Ross have no more than 48 hours to submit a written report online detailing how you did. If your interview falls on a Friday, they are likely to submit the report on the same day, further proof that your thank you note is not going to change your chances of gaining admission whatsoever. My point is that there are far too many variables to even try to game this. If you're dead set on sending a note, do it within 48 hours (preferably 24 hours), and if you don't - that's no big deal either.
To the original poster, I think snail mail will be an overkill. You've already said thank you and it has been acknowledged, put it to bed IMHO.