Those are two separate things, or at least two separate ways to do same thing.
Basically, first one is render of selected objects only. Not isolating them in the scene, but rendering only cutouts of those visible objects. That feature is already being implemented, and will be available soon.
Second one is making objects invisible to camera, but visible to everything else. That can be achieved the way Maru has explained.
I can't imagine how painful would it be to fake render selected feature by plugging most of the scene materials through rayswitcher just to achieve render selected effect, so first way is probably a lot more desirable.