You need to make 2 images, the colour png one and the z-depth one which is essentially  a grayscale image, similar to an alpha channel that is a graphical representation of depth. With a  z-map every pixel in a scene is assigned a 0-255 grayscale value based upon its distance from the camera. Usually the objects closest to the camera are white and the objects furthest from the camera are black with various shades of grey in between.

You then take both into CS3 or similar and add the z-depth image to the "channels" of your png image. You then go to lens blur and set the amount of DOF and from what point you want the focus to be.
There's a bit more to it than that, if there's no tutes about, I'd be happy to do a step by step if anyone is interested.
Any tute I might do would be using photoshop though because that's what I use.
All my DOF images are done in this way, I just posted a new one with the DOF exagerrated to show how much control you have over it using this method.
http://www.renderosity.com/mod/gallery/index.php?image_id=2116396&section_id=&genre_id=&np

Injustice will be avenged.
Cofiwch Dryweryn.