You can mask the background from the parts you would like to be sharp then blur it without affecting the foreground. You can use the mask to build a gradient alpha channel and blur usign the alpha. Bryce will do this for you if you render as a distance mask. then use that mask either for an alpha channel or as an easy way to isolate the background from the image for masking. I know this isn't explained very well, sorry. I would just play around with one or several different masks and blurs for each one.