Is there an easy way to create a "saturation-map" in Photoshop/Gimp? I need a grey-scale version of an image showing where the colors have less saturation (black) and where they have more saturation (white).
You want to convert your image to a HSV/HSB model (Hue, Saturation, Brightness). The saturation is what you're looking for.
Photoshop cannot do this natively, but you might find a plugin that does that for you. The GIMP has this built in, just open the image and select
Decompose from the menu. A dialog will open where you can select the color model to "decompose" to. Convert to HSV and you'll get an image with 3 layers, one of them being labelled "saturation" which is your saturation-map (bright pixels = higher saturation)