Using Mike's technique you can also experiment with something like:

pigment {
  image_map {
    png "Shadow.png" once interpolate 2
    #local i = 0;
    #while (i <= 255)
      #local T = minT + ((maxT-minT)/255)*i;
      transmit i, T
      #local i = i + 1;
    #end
  }
  translate <-1/2, -1/2, 0>
  scale <4/3*CameraHeight, CameraHeight, 1>
  rotate x*90
}

Where "Shadow.png" is your objectless shadow scene
converted to grayscale and minT/maxT are the range
variables for the amount of transparency (generally
0 to 1). CameraHeight is the distance of the orthographic
camera from the shadow plane when rendering the shadow.

Gilles Trans has another techique using an image_pattern
that can be adapted to making soft alpha shadows.
See http://www.oyonale.com/ressources/english/mkofdark2.htm.

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