I know nothing about the technical details of font rendering algorithms, but isn't that just an implementation detail? How hard would it be to write a font rendering algorithm that works when each RGB triplet is a vertical line? And having done that how hard would it be to implement a font renderer that switches between the two algorithms depending on screen orientation?
My gut reaction is that well-made typefaces for digital use, which are often designed with hinting in mind, would lose their benefits. For most fonts, though, it shouldn't make a difference whether antialiasing or subpixel rendering is done vertically or horizontal, and I know you can manually specify your subpixel information in Linux.
Edit: natesm posted a response to the grandparent of this comment with a screenshot of the subpixel orientation selection in Ubuntu.
I've read that the RGB (horizontally) allows better rendering than VRGB (vertically) because (at least in the Latin alphabet) extra horizontal resolution is more often more useful than extra horizontal resolution.
Eg. 'm' and 'w' would benefit more from extra horizontal resolution, while 'e' would benefit more from extra vertical resolution.
