Here is the little eclipse study I did. I was using extreme ambient and surface lighting but the discrepancies between renders and cache calculating requires a cache file, which if it were to be animated would probably look very weird. So a plane is being used providing fake lighting.
Additonally, and interestingly, the atmosphere colours of the moon porviding flare, will paint the atmosphere of the planet very strongly if you use more intense GI lighting. Dunno what that is.
Oh yeah, and ambient light being at 0, or for clouds 1 with colour 0 is important to keep the dark side of the "moon" from glowing.