According to several users in the Rootzwiki forums, as well as other sites, some Nexus 7 units have screen defects including partially raised displays (indicating spots where it is slightly detached from the device where the glue evidently didn't hold), blacked-out corners, and image retention (aka "ghosting"). These issues, of course, are in addition to Google's shipping issues which it says is a direct result of "incredible demand." Such is not unusual in the case of highly sought-after devices, and Google has only recently returned to selling devices directly to consumers. Asus, on the other hand, has extensive experience in manufacturing, but its evident quality-control issues may be the result of a rushed production schedule.

The glass of the screen is evidently raised on the left side of the device in some units, which indicates that the adhesive is losing strength. If you have this issue, it is recommended that you do not push down on the screen to close the gap, since in at least one case, the screen cracked. Even if the glass does not crack, the adhesive will likely give out again, re-exposing the gap between the screen and the rest of the device.

As for the image retention ("ghosting") issue, Android Police reports that other IPS displays also experience this problem, but the pre-production units it tested (the ones released at I/O), the issue was far more severe. Asus responded to Android Police's queries by saying it was not able to replicate the issue on their production (i.e. post-I/O regular production) units. However, indications from our members and those in other forums are that the issue has persisted in the production units as well.

screen washout and ghosting, along with blacked-out corners, image courtesy of Rootzwiki member iPois0n

According to a poll from the XDA-Developers forums, these defects affect units shipped from Google as well as those picked up at retail stores. I have reached out to Asus for comment, and will update you if I receive a response.

[UPDATE]: Rootzwiki member pentafive posted a fix for the raised screen issue here, employing a fix he found at XDA-Developers. It involves taking the the back of the device off and tightening the screws that hold the glass to the rest of the device, so use this fix at your own risk.