Each school has a different policy on this. McCombs at UT Austin automatically allows deferral for 1 year after being admitted. Other schools review each case and have been known to be extremely tough, requiring very large deposits in order to defer, or flat out refusing (even with reasons such as the case of a serious family illness). While there is usually a possibility of deferral, in most cases it is not a sure thing. While these things happen, I would never recommend applying if you think you may not want to attend this year.