I have a methot to make shadows and it works to Objects with alfa channel texture and particle system, but it is in spanish
If you undestad this try and tell me you opinions
http://seyacat.andpalnet.com/blender/envmapshadow.php
http://www.nicodigital.com/xoops/html/modules/newbb/viewtopic.php?topic_id=2033&forum=1&7
http://www.nicodigital.com/xoops/html/modules/newbb/viewtopic.php?topic_id=1987&forum=1#14537
pd: my english suk
pd2: sorry, the tutorial dont have examples but it works