In my two-monitor config the horizontal screen is driven by Apple's mini/DVI adapter and the vertical is (well, was) driven by an IOGEAR USB/DVI adapter using the DisplayLink driver. I've had to turn the DisplayLink screen horizontal because of this bug (the driver still works with no rotation).
Just out of curiosity though I tried rotating the Apple-driven screen and it worked like a champ. I can't really use this setup though because it seems like the screen driven off the mini port will always be the primary, which I don't want vertical, but I found it interesting that the OS can still do a 90 or 270 rotation but the DL driver cannot.
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