While many people who have recently received their Google nexus 7 are having no problems there are a few people who unfortunately have received faulty units. This appears to be an issue with the glass on certain devices and Google not yet acknowledged how widespread this issue is.
Google will of course replace all faulty items but their returns process means that the faulty item needs to be sent back first. That means that owners will be without their Nexus 7s for a while unfortunately.
If you are game for a quick fix, Droid Life has a guide for owners who want to try screwing the screen back in themselves. But this has been noted as not being a permenant fix, as discovered by Shaun over at Lost In Mobile.