Rangefinder magazine has posted a tutorial by photographer Dave Montizambert showing a technique for performing fast background dropouts—i.e. compositing a new background into an image. 

The tutorial uses the "lighten" layer blending mode in Photoshop to realistically insert a new background into an example image. Montizambert also shows how he simulates daylight in a studio using one light. He can then drop an outdoor background into the image behind his model, allowing him to produce, say, a beachside portrait in his studio in the dead of winter.