When you apply, the application will ask when you sat for the Step 2 and to fill in your score. If you don't have the score back, you obviously can't fill the score in. It usually takes over 1 month to get the score back, and you'll probably be submitting your application around early-mid August if you're really on the ball (so if you take it sometime after mid-late July, you should be ok). It is possible that on the interview trail programs may ask you what your score ended up being if they know you've already sat for it. I don't know how common that is. My feeling is that most programs will not care, but that's just a feeling.
The other option (and one I wouldn't recommend ethically), is to just lie. CAS is still pretty basic. Everything is still through snail mail and they don't get any information that you don't send to them. You definitely have to send them your Step 1 score report. But if you don't send them your Step 2 score report even if you have it back, they won't know. And if you lie on your application and omit that you've taken Step 2, then programs will never know you sat for it either. From what I've seen, I'm guessing you can easily get away with it. But that's between you and your conscience.
