I used two different layers to darken the background. Each can be used alone, or combined to enhance the effects.
One layer is a duplicate of the original image with the blending mode changed to darken the image. Add a layer mask (by selection of model or painting with a brush) to reveal the original lightness of the model.
The other layer contains a transparent-to-black gradient set to soft light blending mode. The "background copy" layer mask was duplicated for this layer. This gradient layer makes the darkness increase with background distance.
Scroll through the blending modes of each layer for the most desired effect.

forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=38938033
original image by RayG123: