My method for doing this:
make 2 renders:
1. only the object, set the background as not visible (object properties) in render but still on so you get the correct shadow etc. render with a pure black background. Save this render as an 32 targa with compress on, alpha slit off and pre-multiplied alpha off.
2. render the shadow, so set the background as visible and the object as not visible, render on a as white as possible background. Save this renders as a file you want.
then in Photoshop:
open render 1. go to "channels". right-click the alpha. back to layers, create mask and delete mask (yes apply changes). Now you have a perfectly sharp cutout. tip: select with the magic want tool the empty space, create mask, select mask, Ctrl I. right-click mask, "select and mask" and now you can feather the edges of your object as you like.
open render 2 and place it as background in the render 1 file. use the level tool to make the background of render 2 pure white. create a pure black layer, give the pure black layer a mask. hide your object so you only see the shadow (on pure white). then ctrl A, ctrl shift C, Alt on mask of black layer, ctrl V and ctrl I.
Now delete render 2. Make a background layer (white?) and adjust the pure black (shadow) layer til you have a shadow you like.
Good luck :)
