Rhasan bro is right. I deferred my admission and I was immediately sent a deferral letter in Sept 2010, confirming it. I was sent another deferral letter (similar to the offer letter but mentioning my deferral) to reconfirm my admission for the Fall 2011-2012 session in March.
The policy varies from university to university so you could contact your university to enlighten you further regarding this case.