3ds max mental ray and rapid motion blur

There is an issue in the initially released version of max 9 (the one with mental ray 3.5.2.6) which makes the sun&sky system not behave with rapid scanline motion blur.

The workaround is to turn of the "Aerial perspective". Go to the "mr Sky" light and uncheck the "Aerial Perspective", and you will find the rapid scanline motion blur working fine.