If you run proprietary nvidia graphics driver there is no logic you can guess howto set USE flags with mesa-8 By the way, shouldn't mesa-8.0.0 be masked as upstream developpers announced the normal users shall begin using this new release with mesa-8.0.1 ? Reproducible: Always
x11-drivers/nvidia-drivers does not use media-libs/mesa at all. Somebody neatly pointed that out in the forum thread you referred to.