I think it all depends on the school. Most schools write what they require for defering on thier web site. I think you will have to contact admissions office for details.
I'm also interested in deferring (if I get accepted), but I can't find much info on the web. I'm just going to wait for that acceptence, and then discuss it with the admissions office. I'm sure if you call, though, they'll give you their policy.
I actually contacted one school after I was accepted to ask about their deferral policy because their website had no info. they required a letter explaining my situation and their admissions committee will consider my request. however, i still sent in my deposit since my deadline for that was approaching.. still waiting to hear back from them, but yea i would agree that unless the websites offer info (i remember one school said they allowed deferrals for physical reasons and you need a doctor's note saying you cannot begin school with the rest of your class), call the ad com after you get an acceptance from them. =) good luck to you all!