How to implemet DOF via GLSL

Hi there
I want to implement depth of field effect via GLSL. is there an example or article on the internet about it?
thanks

google search gave me these useful links
http://artmartinsh.blogspot.com/2010/02/glsl-lens-blur-filter-with-bokeh.html
http://www.gamedev.net/community/forums/topic.asp?topic_id=487503
http://encelo.netsons.org/blog/2008/03/23/i-love-depth-of-field/
http://encelo.netsons.org/blog/2008/04/15/depth-of-field-reloaded/