Ron has a good comment up on the Physical security post. It deals mainly with various aspects of the wireless discussion and the use of reserved vs. common bandwidths. It also touches on something I should have expounded on better.
Any of the items I mentioned might be too extreme or not extreme enough depending on what the relative risk level is for a given system. For one system (on the physical protection level) an electrified fence with constantine wire, an armed guard and trained guard dog may be appropriate. For another a padlocked plywood door might do.
One of my key points from the post was that certain design factors of the physical system can be considered mitigating factors for information security risks related to the ACS.
Alex at Riskanalys deals very well with mechanisms of determining how significant potential issues can be. Hopefully some time in the future he an I might be able identify subsections of impact modifiers, threats and controls specific to DCS and SCADA.