If I decline admissions at a program that doesn't allow deferment, will I be accepted next year? I've been accepted to a top 20 Computer Science department with a RA, TA and fellowship. I decided to defer my admissions a year due to unforeseen circumstances. I was informed the school does not allow deferment, and my adviser told me to simply reapply next year. If I was accepted to the university this academic year, should I be able to get accepted next Fall? My application will be the exact same as it was this year including my recommendation letters, personal statement, and research experience.
I'll address the chances of being admitted again next year at a school that accepted you this year.  Certainly they aren't 100%, or even 95%, since if they were that high, then the department would save itself the trouble and simply allow deferrals.  As Aeismail points out, it also depends on the strength of the applicant pool, and that can fluctuate from year to year.  For example, in a bad economy, or if the department hires a star faculty member, it can become harder to get admitted.

Department admissions procedures vary greatly.  In some departments, having strong support from a single faculty member may be enough to guarantee admission.  That is not true in my department, and I'd estimate that the average chances of being admitted again in this department are about 75%.  (Don't take this figure too seriously, but it's a better estimate than 50% or 100%.  It's based on experience and thought experiments rather than data.)

Roughly, the top 25% of the class is admitted quickly and easily, with little debate.  The next 25% involves more discussion, but the answers are probably pretty consistent, and the main issue is ranking for things like fellowships.  It's the bottom half that involves increasing amounts of randomness.  The number of plausible acceptances - people where nobody would think it strange if they were admitted - is substantially larger than the number we will actually accept.  Near the dividing line, there are a lot of people whose applications are really pretty comparable.  We arrange them in order, after a lot of discussion, and draw lines for admission and waiting list.  The order is not completely random, of course; the stronger your application is, the higher you are likely to be ranked.  But there's a lot of arbitrariness to it, since it's a matter of small distinctions.  The trickiest cases are the applicants who have notable strengths and weaknesses, and their positions depend sensitively on who is on the committee: getting a high ranking depends on having some committee member who strongly supports your case, and nobody who is strongly opposed.  (Admissions committee are often risk averse, so opposition counts more than support.)

Overall, I'd guess that as many as half of the people in the bottom half of the list might have been replaced with someone else if another committee had examined the applications from scratch.  That would correspond to a 75% overlap between the decisions.

Of course, this figure is pretty much meaningless for individuals.  If you are a star, then you will definitely be accepted again.  If you were the most controversial and lowest-ranked person last year, then your chances of being lucky next year as well are not good.  The truth is most likely to be in between.