Antony Riakiotakis just made the first commit for the new Depth of Field and 'Screen Space Ambient Occlusion' in the 3D viewport. See it to believe it!
Viewport compositing - first code
This commit introduces a few ready made effects for the 3D viewport
and OpenGL rendering.
Included effects are Depth of Field, accessible from camera view
and screen space ambient occlusion. Those effects can be turned on and
tweaked from the shading panel in the 3D viewport.
Off screen rendering will use the settings of the current camera.
Here's another example - one Andy tweeted earlier this week. What do you think?
— Andy Goralczyk (@artificial3d) February 10, 2015