You could try the following:

Select everything in your objects list to the right - except of the sunlight object. Now that you have all, the camera, the objects, the layers, the ground selected as one, zoom into the side view window and grab the Z axe and pull it upwards a few small steps.  Mark a small render area right including this fog line to make a quick render and check, if the line is still visible.

Repeat this upwards movement in small steps - render again - move a few pixels - render again, until this line hopefully disappears.

Before:
img467.imageshack.us/img467/8316/fogcutlinepic1hj2.jpg

After:
img467.imageshack.us/img467/4008/fogcutlinepic2yh1.jpg

This way you only make short movements and it doesn't affect the angles of lights/shadows and the camera view/angle/position are staying the same relating to your scene, but this strong perfect cut line could disappear.

Not sure either, if this would work with any atmosphere setting or style, but it's worth a try ...

Why Vue is doing that ? No idea - but it could be something, the programmers could take a look into for one of the next updates.