@StoneMonkey: Yeah, it just uses the current texture unit. You could simply render your scene to a frame buffer texture object and apply 2d effects to that though.
I've finally got the robust shadow volume technique that nVidia outlines
here working, and what I really would like to do with glsl, is soften the shadows. I'm not sure how to go about it yet though. I mean, you can't really "blur" the stencil buffer. I've tried "clusters" of lights for each light source, but that's really slow for a scene with a high polygon count. I was thinking like, setting the alpha of each vertex in the shadow faces based on the distance from the center of the object. Then again, you aren't actually rendering to the color buffer, so...
