I have that same problem(?) too but I'm not gonna nitpick over something like that. I don't feel like going through the process of sending it in and waiting for a replacement. the wait for my first nexus 7 was bad enough. the screen issue does not impact the performance of the nexus and does not seem like its gonna get worse. just my two cents on this issue
parth.s, my N7 is exactly as you described yours. I'm going to give it a week or two and see if it worsens. If it does I'll pop the back off and tighten the screws. Only if I can't fix I will I send it back. I've noticed that after 4 days of use it is starting to creak here and there. It is subtle but annoying.