Google Nexus 7 has separation problem with its screen
A few Google Nexus 7 owners decided to copy this maneuver and found upon removing the back cover of their unit, that the screws were quite a few turns away from being sufficiently tightened. While getting a screwdriver and turning the screws (no, we don't recommend the Oppo Finder for this operation) helped some, it didn't solve the problem in every case where there was screen separation.
request a replacement unit. Now that the tablet has started shipping in number, you probably won't have to wait terribly long for a replacement. And we would try to avoid saying anything about having a screw loose because someone might take it the wrong way.
If you are the type that must fix it yourself before surrendering to a technician, you can find directions on how to remove the back cover and tighten the screws at the sourcelink.
source: XdaDevelopersForum via Phandroid