I don't have much spare time these days so I'm raiding the vault for this post :) These are some old post process depth of field tests I did ages back. It's quite an expensive method so I've held off implementing it in the engine so far. I rendered the source colour and depth images in Mental Ray and used Render Monkey for these tests. The images below show the effect of moving the focal plane through the scene.
(click for hi res versions)
The source images: A linear space colour image and scene depth, both 16 bit.