This hour, you've learned to consider the physical environment as an integral part of a holistic approach to Linux security. The basic steps to be taken are
Lock away all systems in isolation when possible.
Prevent power cycle issues by disabling reset and power buttons, securing the power cable, and installing battery backups.
Lock floppy and CD-ROM drives or remove them altogether if they aren't needed.
Install a cable lock of some kind to prevent your entire box from being stolen.
Lock your case as well.
Keep an active list of those individuals who have physical access to the machine in question.
Be aware and be vigilant!