Play on full screen in slow motion 
http://mond.own.cz/blender/clouds/el.clouds500.mov
whole show was done by side effect of SSS (no lights setup or animation)
and ones without SSS
http://mond.own.cz/blender/clouds/cloudsNOsss.avi
I had a plan to made volumetric clouds, how you can see i lost a power very fast But I like idea that I tried. An the end - nothink new, you can see kombination some of done (on this site), but from my view its simple and relativly fast accordant to result.
Just - Camera, Plane and two Empties
- Plane infront of Camera duplicated by Array is control by Empty
- Plane using just one Cloud texture and one Blend texture to control limits of high, bouth control by second Empty
(+ lights Sun and Hemi)
Thinks that I like on this idea:
- easy control quality (details of clouds)
- planes generated only for camera view (minimum objects = fast render)
So . . .
- objects close to camera can take more smooth intersectionwith cloud
- clouds far from camera has less layers but who cares for less details that are not recognizedable from camera view
- easy to take fast preview result by changing array count
And a result that you can see is SSS side effect (Im beginner so difficult to say way for me).
I will pleasure if somebody will find it usefull for his project or will do next step to get better volumetric result. Good Luck
And let mi know how you did it.
Thanks
.blend file
http://mond.own.cz/blender/clouds/el.clouds.blend.zip