I have to say that I have this exact same issue. The top part of the phone can definitely be pushed down, and makes a noticeable clicking sound upon doing so. It doesn't appear to affect the use of the phone in any way, but it is rather annoying for something that is supposed to be a solid piece of polycarbonate.
Too late to switch it out for a new one sadly.
While I don't particularly mind it, I do wonder if it will get worse in the long run. Is it coming apart slowly? I don't remember this when I first got the phone, but then I also don't remember specifically pushing in the top of my phone either. Will it affect the touch screen performance one day? I certainly hope not.