Can you imagine how that looks to admission committees? You telling them how you feel you've been called to be a dr., you really want this bad, etc. etc., and then you basically are saying well, I want it, but not enough to go to THAT school. I would imagine it is VERY rare for someone to get away with this!
I would think that the reason why would be a factor as to whether or not it is acceptable. I know my school does not regularly grant deferred admissions after May 15, so if there was a reason such as that, where you needed to take a year off unexpectedly, perhaps it would be more acceptable to schools the second app year. But if you are just trying to get into a better school, I don't think it would be looked upon favorably.
While I agree it's risky, I do know someone who has successfully accomplished that task. He was accepted at Davis and decided he didn't want to go. The following year he applied again and got into UCSF - where he is now. Don't know the specifics of the reasons given to the adcoms, but he was successful. Good luck!