Quote - I'll post one when it finishes baking.  May be a while.  Meanwhile see link to one in my gallery.

The buildings and street look wonderful in that render, the machinery in the foreground looks too clinical though and the sky doesn't show any gradient or variation.

It would be a good scene to experiment with the sunsky parameters.
Have a look at the abstract of a research document on the Perez Sky Model. I'm guessing the parameters a-e in this text are connected to the aconst-econst parameters in the sunsky light.

======== Abstract
Perez et al. developed a five parameter model to describe the sky luminance distribution [24]. Each parameter has a specific physical effect on the sky distribution. The parameters relate to (a) darkening or brightening of the horizon, (b) luminance gradient near the horizon, (c) relative intensity of the circumsolar region, (d) width of the circumsolar region and (e) relative backscattered light.
 

========= example sunsky

If you want really blue skies, try this:

AttributeBegin

LightSource "sunsky" "integer nsamples" [1]

            "vector sundir" [0.70000 0.400000 0.700000]

        "color L" [1.000000 1.000000 1.000000]

        "float turbidity" [2.000000]

        "float aconst" [0.500000] "float bconst" [0.500000] "float cconst" [1.000000] "float dconst" [1.000000] "float econst" [1.000000]

        "float relsize" [2.000000]

AttributeEnd
 

Software: OS X 10.8 - Poser Pro 2012 SR2 - Luxrender 1.0RC3 - Pose2Lux
Hardware: iMac - 3.06 GHz Core2Duo - 12 GB RAM - ATI Radeon HD 4670 - 256 MB