OS X 10.8.5
Current Blender 2.6x
Short description of error
It seems while having shadow ray disabled a mesh light object still blocks some of the light from the world light.
It feels like the direct light is there but indirect light is blocked if I read this right.
The screenshot shows the problem. The images on the right show the difference between shadow and diffuse ray visibility flags.
I do not know if this is a bug or if this is a missing function because with a point light all works and no light is being blocked from the point light.
Exact steps for others to reproduce the error
Disable shadow visibility for a mesh but it still blocks some of the sky light.