Here is the unity package for my 2D per-pixel shadow system. Enjoy:)
https://dl.dropboxusercontent.com/u/17249329/2D%20Shadow%20Test.unitypackage
You need unity pro for it.
You can read more about it here
http://forum.unity3d.com/threads/180769-2D-dynamic-soft-shadows-(will-release-source-later)