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While printing and scanning, gray (and black/white) colors get saturated a bit. on the contrary, vivid colors are lost their saturation.
for this reason, saturation should raised only high saturated values, and decreased low saturated values on a scanning process.

The smartcurve plugin is able to do this operation, but Photoshop can't do this in the default. you need to download HSB/HSL plugin from adobe (when you using CS4 or CS5), then change channels from RGB into HSB or HSL, and use curves to the S channel.