I find it fascinating that the human eye can pick up on that kind of effect, considering the screen is driven at 60Hz (16.7ms). I haven't noticed it on the first-gen iPad [1], but it suddenly becomes apparent when turning some desktop screens into portrait mode. There must be something about horizontal movement that the eye picks up on. Either that or we're completely desensitised against vertical refresh because all screens we've grown up with refresh that way.

[1] I can only assume the new iPad's LCD is more inert in some way? Smaller pixels, slower crystal?