Unchecking 'Enable Secondary' in the cloud layer is how you'll stop it casting shadows.
'Enable primary' is the cloud forms and shape visibility.
'Enable secondary' is the layer's affect on the atmosphere and planet surface visibility.
Here's default cloud settings:
[attachimg=#]
Here's no 'primary' ***:
[attachimg=#]
And, no 'secondary':
[attachimg=#]
*** There is a problem(bug?) with using the 'enable primary' button, though. If you select to disable primary(maybe you just want the rays and not the clouds, for example), then if there is a reflective surface/water surface in the scene, that surface will still reflect the 'primary' element of the cloud layer, when its isn't visible in the atmosphere.
