I'm not sure if its software or not. I used to use the Portrait software. It came with my Viewsonic LCD but I really had a hard time with it and it was fuzzy for me.
Maybe it was a difference in the cards themselves.
I was thrilled when the new nVidia drivers came out and supported rotation and its crystal clear for me. Ms. PacMan is beautiful on my rotated LCD!
(oh, BTW I play it rotated via the driver, not MAME in this case

)