How to disable backface-culling? I'm trying to create a mesh with faces that are semitransparent in such a manner that I can see the back faces through the front faces. 

Setting the transparency of the material allowed me to see a completely different mesh if it were behind the "transparent" mesh, but it didn't allow me to see through to its own faces. 

The mesh as a whole got transparent, but the faces all kept an opaque appearance (if that makes any sense).
Using a simplified version of the node setup from my answer here, you can do this with cycles (click on the image to get a bigger version):