Since I read this thread, I been playing with just what lsd suggests. As you can see in the pic.

It's a combination of inverted distance mask and inverted object mask used as a filter mask in Photoshop. It's not perfect, most probably because the objects are small, so the distance mask doesn't have a big range as it would have if it were big. And the colours on the blurred objects are off. But, it's a start. :-)

BTW, for an even better of example of selective focus, take a look at this photo in my gallery. ;-)

-- erlik