If you applied Early Decision, then the program has until 9/25 to make a decision. You may not get a rejection letter because they can roll your application into the regular admissions process if they still like you. You might get a letter telling you that you are still under consideration.
If you applied to regular admission, then some programs will send you a rejection right away if you don't meet their minimum standards. If you haven't heard anything yet, I would think that is because you are still under consideration. You might not hear anything until they offer you an interview and some of them interview all the way into January.
I know it is hard to wait!