This blend provides a convincing animated sky with wispy clouds. A few highlights:

- Drivers are used to make the built-in sky texture follow a sun lamp.
- The world texture is modified to make the sun directly visible in the sky (since Cycles doesn't show it by default)
- The clouds are on a large plane instead of in the world shader. This enables them to be lit by the environment (including the sun), and to cast shadows.
- For the clouds, I created a custom "Omni Shader" that scatters light in any direction without bias. At first I thought a 50% mix of the Diffuse and Translucent shaders would accomplish this, but that combination is biased against scattering perpendicular to the surface, and the result shows up as an ugly artifact.