I met a couple of people who deferred at MIT and at GSB and I have a friend that could do so at Harvard. I don't know the reason for the MIT student, and the GSB one I don't know also, but his in my class.
I know the history of my friend with Harvad though. He had started a business with some friends and he thought that he could let everything with his partners; however, the business developed much faster than they were predicting and he decided to postpone. Harvard let he in with a updated Resume and an Essay telling how was his experience during this year. This person graduated as the Top Student from a Top 3 engineering school in Brazil, worked for some big consulting companies, and is perhaps the most intelligent (analytical, emotional and social) person I have ever known.
He was dinged by Kellogg at the first year he applied.