Hey guys,
I've always had an image map or a power fracter plugged into the default shader's luminosity input. If its just black, it won't light up the scene...it needs color. The values I've used range from 2 to 10 depending upon how much light is needed in the scene, and usually indirectly proportional to how much sunlight there is. For example, if I want a super hazy scene, the I set the luminosity pretty high (4 to 8 or so) and the sun's strength low.
But here is the kicker: I am not using the default sun. I am using a light sphere thats pushed back out into space but still within the background object and scaled appropriately. The brightness needs to be bumped up bigtime, usually to 2,000,000,000 or even more.
The reason for using a light sphere is, there may be a bug with the default sun. At random times, it just does not want to shine through the background object. You can see it, but it doesn't illuminate the ground. Its inconsistent and weird, so I disabled it, and ended up using the light sphere which has given me no problems.