I got an iPad 4 just before the New Year and, while I absolutely love it to bits, I have noticed what looks like a single speck of dust under the screen toward the top left hand corner when the device is held in landscape orientation and displaying a white background. It is definitely under the screen as I cannot see it when held in portrait mode and there is no obvious mark on the screen when it turned off. It is only visible when viewed head on and if I tilt the screen toward or away from me then it seems to disappears. It is the kind of thing no-one else but me would notice and even if they did they'd probably shrug their shoulders and ignore it. Me being me though, it is starting to irk me a little as I know it is there and find myself constantly looking for it!
I must admit to being rather surprised to find dust under the screen as I've had lots of devices with LCD screens over the years, including a Samsung Galaxy S3 smartphone and PS Vita, and have never noticed any dust or specks behind the screen. I always assumed that was because they were made in dust-free environments and certainly never expected to see it in a premium quality Apple device. I Google-d this issue last night and to my surprise found that it isn't such a rare issue with iPads after all.
I guess I'm going to just have to learn to ignore it because there is no guarentee that the replacement won't be the same... or worst!