In this HD youtube video I use Photoshop's 'Blend if...' function to create split tones and rich-looking sepia photographs.
You could use the same technique to de-noise an image just in the shadow areas of a photograph all under interactive preview control.
UPDATE 4/20/2009
Read the comment below, then click me
I've been using this for split-toning for a while, and it can work really well.
I don't duplicate the pixel layer(s), though; I just add 2 Hue/Saturation layers, and set the Blend If sliders on the upper one. That still gives the full range of control over the underlying brightness-based data, though it does sacrifice the ability to handle channel-specific blends (that is, you can't do a Blend If on the underlying Green or Blue or Red channels independently).
For most work, this is a lot faster and easier. While an Action can speed it up, it also doesn't require duplicating (or merging and duplicating) the pixel layers.
Nice video.
Hey, Kevin, nice to see you.
Interesting.
It never occurred to me that an adjustment layer would even offer a 'blend if' slider.
UPDATE:
http://www.deke.com/content/martini-hour-010-in-which-deke-apocalyptically-imagines-a-world-layers-inside-channels#comment-2393