Nexus 7 screen is flickering
There just was a Blog on Land of Droid covering this issue. The author summed up a few conditions under which this "flicker" occurs:


back-light is dimmed under 30%
a weak WiFi signal
the device is accessing the net


According to that report, these 3 things are "AND-connected" (i.e. the flicker occurs when all of the 3 conditions are met). He's also hoping for a fix to come soon.

You might as well want to read the comments there, some seem to indicate possible solutions (including flashing a new stock ROM), but so far none of them are yet confirmed.