Will be going back to Siggraph today.  I will sit through a lot more of the 6.5 demos.  But as dsr207 and Mazak pointed out, click that really tiny icon on the light control to have it effect the cloud.  Works great!

As far as what e-on was saying about thunder storms - using a combination of dark colors and negative lights, you can get some really stormy looking clouds.  

When I get a chance, I want to try this out for fire and smoke...

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