It depends on the school. </what everyone else said>
I considered deferring from Wash U last year for personal reasons. I think their deadline was mid-June. You had to write a letter requesting it. I called up and talked to one of the deans. He said that Wash U generally grants deferral requests because they don't want people there who don't want to be there - better that you figure out medicine isn't for you during your deferral year than during MS1. And they're supportive of other pursuits (grad school, travel) as well.
My personal situation ended up working out in such a way that going straight to school was the best choice, so I didn't end up requesting a deferral, but if things had gone differently it might have been the best decision for me.
Two pieces of advice he gave me that might be applicable to people reading: first, if you're considering deferring, and your admission/scholarship/research advisor/whatever isn't already finalized, don't tell anyone. Some people (like the dean I talked to) won't care, but it's fairly likely that some people will think you're not committed to medicine, and deny you the admission or scholarship or place in a lab or whatever it is. Obviously don't lie or back out of any commitments, but don't go around advertising it either.
Secondly, deferring due to finances is not a good reason, unless you're going to have to declare bankruptcy or something. For the vast majority of people going to med school, your post-med-school earning power is going to be so much higher than your current earning power that it just doesn't make financial sense to put off med school - even if you have to take out private loans to cover your car payment (or whatever), the interest you accrue on those is still going to be outweighed by the increase in lifetime earnings you get by starting school a year earlier.
I have no idea how much of this applies to you, tigress, but I thought I'd put it out there for general consumption.