I played with this a while ago as well, and after numerous attempts I was never really satisfied with the results that Bryce gave me. I found it much easier, faster, and got better results if I wanted to use DOF in a render  by rendering seperate masks in Bryce. I would them combine them in postwork using blur effects in an image editing program such as Paintshop Pro or Photoshop. Depth of field generator pro looks promising though, I've never seen that one before.