It would affect subpixel rendering of fonts, but I think the iPad doesn't do any subpixel rendering. Not 100% sure though.
It does create another strange effect, though. Since the screen effectively redraws sideways if you hold the iPad in portrait, scrolling vertically quickly makes stuff look a tiny bit distorted; horizontal lines look a bit slanted (like taking a picture out of a car using a cellphone camera, but much less so).
You're right about not doing subpixel anti-aliasing. It would need to change the direction of the subpixel anti-aliasing on a switch from landscape to portrait, which would presumably lead to strange visual effects.
That's interesting. I guess Apple decided not to enable vsync? I don't recall seeing tearing playing games on my 4S though, can developers elect to turn it back on?
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.
It does create another strange effect, though. Since the screen effectively redraws sideways if you hold the iPad in portrait, scrolling vertically quickly makes stuff look a tiny bit distorted; horizontal lines look a bit slanted (like taking a picture out of a car using a cellphone camera, but much less so).