Thanks Mike. I checked my settings and it looks like they are both (c2 &c3) set the same. The ambient and sky presets were identical in either version.

There is a new option in the render tab for Sky Light. I checked this and raised the intensity to 100% but it is still darker. This is likely the issue but if I like how the scene is lit with CS2 GI, shouldn't CS3 be close to the same or do you have to always tweak the scene when a new version comes out?

Here is the C2.1 render...