Australian photographer Jeremy Daalder has a great tutorial that dicsusses techniques for imitating classic photographic effects and film types in Photoshop.

“The Bleach Bypass is a film laboratory technique where, by skipping the bleach stage in the color processing sequence, silver is retained in the image along with the colour dyes. The result is effectively a black and white image superimposed on a color image. Bleach Bypass images have increased contrast, reduced saturation, often giving a pastel effect. This technique (combined with digital grading) is VERY popular on shows like CSI etc.”

Website: Jeremy Daalder - Simulating Classic Film Effects in Photoshop