Under the shading panel, try checking "Tangent Shading".
Doing that will make it look like this (that's Suzanne behind the plane):
shadows.png
You might also want to try selecting raytrace transparency, turning down the alpha and also turning down the gloss to a value where it fully blurs what's behind the curtain so you can't see it (or you might want to leave it up a little bit for a teeny bit of transparency in the curtain.
By the way, that was achieved without tampering with any other settings, so maybe try from scratch or adapt it to your current material setup.
P.S. Curtains look a tad thick... Perhaps you've done that deliberately while trying to get SSS working, though.
You do not have the required permissions to view the files attached to this post.