Yeah, lots of other people have commented on that too... It seems that the graphics hardware does some kind of post-hoc rotating, rather than thinking natively in the rotated resolution. Either that, or there's a lot of added processing power to add scan lines, as opposed to adding horizontal res? Anyway, yes, it seems to happen to more people than just you. I noticed it briefly, too, when I tried rotating my notebook display.
Nope, mainly because ATI and Nvidia writes the Windows driver. Apple writes all the drivers that ship with the OS and that includes the GPU. Though I believe they borrow some from the OSS (like GIMP print).