According to one Gizmodo reader who attempted to fix the problem on his own, the problem is related to a layer of material behind the LCD used the scatter the LED backlight. He suspects this material may become misaligned during shipping, causing some areas of the backlight to have an uneven color temperature.

Whatever the cause, information on the issue appears to be in Apple's internal support system. Apple support technicians are told to tell customers complaining about the issue that slight variations are normal. If the customer complains further, the tech is supposed to get contact information so the customer can be contacted about having the display panel replaced "in approximately three weeks." Those asking for "immediate resolution" should be offered a refund and not a replacement.

While the problem also seems to affect 21.5" iMac models as well, the Apple support information doesn't address those models.

Since Apple specifically mentioned a three-week window for an expected fix, you might want to hold off buying one in the meantime. Online orders quote at least a three week wait, so Apple may be holding of shipping machines until the fix in place. Still, for users currently experiencing the problem, another month-long wait or a refund probably isn't a very satisfying solution.