I noticed it on an iPad at the Apple Store. I was scrolling through photos horizontally in landscape mode when I noticed the vertical edges of the photos became slightly tilted. It is as if the top of the photo could not catch up with the bottom. In Portrait mode, when swiping through app pages, icons seemed to stretch in when moving in one direction, and compress in the other.
I did not try another iPad and therefore cannot say if it is a common issue. iPad 2 does not have this issue.
My sister could see it (barely) after I told her. My niece couldn't. Therfore It is possible that some people are more sensitive to this phenomenon than others, just like some people can see the "rainbow effect" on DLP projectors whereas others cannot.
I'll check it out on another ipad when I have a chance to go to the Apple Store again.