I tried the following in CS4 with a scan from a b/w negative which had a grainy sky:
I first inverted the scan and applied the levels tool to have some sane grey values (Background layer).
Then I duplicated the background layer and applied some aggressive denoising (Denoised layer).
Then I duplicated the background again and applied various filters, like emboss, find edges, blur and such (Mask layer) to
get a layer where the originally detailed areas were black (=masked out later) and the big low-detail areas, like sky or house walls were white.
(depending on the filter it can be vice versa, but you can invert later ...).
I created a layer mask for this mask layer and selected the white tones to be applied as mask.
This mask I dragged over to the aggressively denoised layer and Voilá-there were my details back.
The intermediate mask layer I simply turned off.
Has anyone similar experience or better ideas on how to mask out detailed areas when denoising ?
Input welcome!