This is the VEX Volume Cloud shader's code wrapped up into a vop, so you can jump start creating a custom volume shader.
Simply create a new VOP shader, put down a rsIlluminateVolume vop, create a "density" parameter wired into the density input. Wire the "colour" and "opacity" outputs to Cf and Of respectively and you have the equivalent of the VEX Volume Cloud shader.
From here you can modify the density value going in (such as multiply it by a noise), or have fun re-colouring the output.
NB: Of course.. this can totally be done with RMB: Create New Vop Type... Didn't need to create this afterall!! Doh!
NB again: Took this out of the trash, since the Create New Vop Type functionality is broken in some versions of Houdini.