I don't think that adding noise by itself will help banding issue.
Gaussian blur, followed by add noise does pretty well on plain wall, but the amount of blur required would totally mess up the other details in the image.
I think the ISO used and amount of jpeg compression might be problem areas to consider improvement.
I boosted contrast in levels to exaggerate the banding comparisons.
If I used this method on selected areas, I would blur, then only add enough noise to simulate the original image. (Similar to what is done in some of the skin softening techniques)

forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=24970667

original image: http://caleido.deviantart.com/art/who-s-there-65801571