I can reproduce this in 151 version.  I noticed another strange anomoly with this.  When I set the horizon to 89 degrees, the render goes a LOT faster.

Weird.

The image shows the result.  I jacked up the light contribution from the skylight to 300% just to get it to be as obvious as possible.  The left image is with the horizon set to 90 degrees.  The right is set to 89 degrees.  There is no other difference in these renders.

-Kix