When videotaping, make the background brighter than your subject. With a manual iris you could adjust the exposure to correctly light your subject and create this effect.
The iris is used to adjust the aperture of the camera's lens. Opening the iris lets in more light, to increase the exposure, or brightness, of the image. Closing the iris creates a darker image.
If you already have the footage use the 4 or 8-Point Matte Filter in FCE. Put identical clips on top of each other, apply the blur (or darken up) to the lower clip, and use the Garbage Matte to reveal only the area you want to blur or darken on the upper clip. You could also try the free iSilhouette plugin:
http://www.chv-plugins.com/cms/Fx-Script/iSilhouette/iSilhouette.php
There's a blur technique you can use too...this is a tutorial for premiere and final cut:
http://thedvshow.com/editbay/premiereblur/