As far as working with DOF in postwork, I use various methods utilizing blurred layers with gradient masks. For those of you not familiar with Photoshop, it's not difficult and you can use the DOF alpha mask renders in Bryce to enhance the accuracy.

There are a few images in my Gallery with successful DOF's.

There is also a Program that Agentsmith posted a thread to some time ago.
It's called Depth of Feild Generator Pro.

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Ockham's razor- It's that simple