It's not tearing, it's just a slightly slanted picture, because the lines on the left (or right, depending on how you hold the iPad) redraw a tiny bit earlier than the ones on the other side of the screen.
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?
I think one of the reasons why we don't notice it on desktops is because there are so few vertical lines. Basically, it's the window edges where it might be noticeable, when dragging windows sideways.
On the iPad, on the other hand, there are tons of horizontal lines in a table view, for example.
Also, I thought about the 60 Hz thing, and I think high frame rate is a bit of a red herring. After all, the lines actually are slanted; the higher the frame rate, the fewer degrees they're slanted at the same scroll speed, but whether we notice the slanting is independent of the frame rate.
You're right, I'm able to make out the effect on my desktop screen when dragging windows from side to side. I guess we only scroll vertically most of the time, not horizontally, so it doesn't come up. That suddenly is no longer true when you rotate to portrait orientation.
