The same effects can be created in the digital environment using "filters" in CS5 or the Silver Efex Pro Plugin
Base colour image
Basic monochrome conversion
Red (sky) and Blue (land) split Filter
Yellow (sky) and Green (land) split Filter
The examples above demonstrate the basic rule of thumb in that the same colour filter as the object in front of lens then the object colour goes lighter. Opposite colour [green to red, red to blue, etc] darkens the colour in front of the lens. So a blue sky photographed with a red filter makes the sky go dark (not the clouds) and trees and grass photographed with a green filter makes the greens lighten. The use of the filters in post processing rather than when shooting allows the amount of filter to be added to be adjusted between 0% to 100%.
The split filter examples require the use of a layer mask to "paint" in the correct filter to the area of the image as required.