How could a one-sided material be achieved with the Cycles renderer?
I would like to create a plane that is completely transparent from one side, and a mirror from the other side.
It would be cool if Cycles/Blender simple supported backface culling like the game engine. But I guess it doesn't?
How could something like that done with nodes?
In cycles it's a simple matter of using the Backfacing output of the Geometry node:
The Backfacing output gives us white (a value of 1.0) on the side that's facing away, and black (0.0) on the side that the normal is pointing
(This is what you see if connecting the Backfacing output to an emission shader as a sort of viewer node)
So connecting this to the Fac of a Mix shader with Glossy (the mirror) and Transparent shaders will give the effect you're looking for.
You can do this in Blender Internal renderer exactly the same way only with slightly different nodes.
As for backface culling in the viewport, there's a check-box for that in the Display Panel of the Properties Shelf in the 3D view:
Which would of course make faces on the other side of the normal direction transparent by definition:
This is done using the Backfacing output socket of the Geometry node.
Backfacing
1.0 if the face is being viewed from the backside, 0.0 for the frontside.
Use it as the factor of a Mix Shader node and connect your shader along with a Transparent BSDF shader node.
Connect the Front/Back output socket Geometry node directly to the Alpha of the Output node.
Maybe there's an easier route than this node setup. This node tree mixes full transparent for that portion of the mesh that is backfacing, and the rest visible 
