i think that displacement has to do with the overall scale of the "cloud" object.
if the cloud is too big the scale of the displacement texture should be big enough too, else if the camera is set far enough from the cloud formations, the results won't be visible.
i'll do some testing about that.t
the smoke rings are displaced torus objects.
this is the file that should be merged into the cornellbox examples scene.
if you set physical sky on, you will get something like this:
File comment: 5mins@5s/p
smoke-03.png [ 392.61 KiB | Viewed 1233 times ]
edit: setting "normal smoothing" off in displacement settings, will look much better
this is an example of absorption & scattering values set to 1
smoke-05.png [ 392.48 KiB | Viewed 1189 times ]