hey - funny to see my posts from a long time ago-- now i'm singin' a different tune:
you can _definately_ defer at a most schools [can't say with certainty about all], but try to let them know as soon as possible after acceptance [this may not be possible if you got in late from the waitlist - but this would go for med school people too]. you are right, there is someone who defered entrance into our mstp class - she is teaching out of the country for a year and will entering with your wave. most schools do want to know what you are doing, just so you aren't simply hanging out at home or something.
furthermore, i think if you can defer you should definatley do it - there is no better time to do it than after college, and there is so much to gain. the md/phd is a lifetime program, and a year isn't going to make a difference with regards to starting. on the other hand, if you get a unique experience, or just take a break for that year, who knows how it could enrich what you do next?