This is something that's bugging me about the iPad. Since the screen is 1024x768 (and TFTs are typically manufactured in portrait), I wonder if the pixels are oriented vertically rather than horizontally when you're in portrait mode. If so, the font rendering for books could be horrible compared to if you had it in landscape.
As far as I can tell the iPhone/iPod Touch don't use subpixel rendering, just antialiasing. The display is dense enough that (to my eyes at least) it really doesn't matter. Likely this will carry forward to the iPad, which has a slightly less-dense display -- ~130dpi vs. ~160dpi -- but which will also be viewed from a little farther away.
Would it be possible in the future to supplement our existing font formats to have two versions, one for subpixels one way, and one for the other (so that the OS could transparently switch between which version to use)? Or am I misunderstanding how subpixel fonts work?
That depends on if it's a bitmap font or not. If it is bitmap, then it's already subpixel rendered, and you would need two different bitmaps depending on orientation. If it's truetype or similar, you would need a font renderer that was capable of subpixel rendering, and it would then have to render it differently depending on orientation.
However, subpixel rendering works because you effectively triple the horizontal resolution. If you flip your screen, you now have normal horizontal resolution, but triple the vertical resolution, which is less useful for rendering fonts, and it would of course also not look the same as the horizontally subpixel-rendered font.
So for devices like that, it's probably best to simply not do it and get a dense enough display that you won't need it.
Also, does anyone know if subpixel rendering works on OLED screens?
Anyone got any ideas/insight on this?