In the OS 9 days, there was a low-level disk setting that told the disk whether or not it should automatically mount. OS X does not have this option and, like many good things in the system, it relies on old-school Unix.
I actually use this trick fairly often (and, it can be considered an adjuct to my August 2006 MacTech column "GUI up Your Script). It's great to be able to clone the system disk, and then 'hide' it. The benefit of this is that if the primary drive fails, a cloned disk will just boot, without the operator knowing what happened.