Do a search for transparent and you'll find Mael's post:
From: Mael
Subject: shadow on transparent background [12.6 kbu]
Date: 2 Jan 2003 03:03:25 EST
to answer http://news.povray.org/3e0cb100%40news.povray.org i found a way to
simulate shadow (or semi transparent shadow) on a plane (or on any object)
with a transparent background by using the projection patch in mlpov.
