Lady Lee said:
Whenever I do work in there I turn off the computer and unplug it. Then I touch the case and ground myself before I stick my hand in to fix something.
This was the essence of my original musings to Jourles. If the case is unplugged, it's not grounded unless there is some other grounding mechanism in place.
This can be in the form of direct contact with something else that is grounded. (e.g Laying on a workbench ground pad; being installed in a grounded server rack, etc.) orby being physically connected to a good source of ground. (e.g. By being alligator clipped to a ground source, etc.)
Leaving the case plugged in is one other way to achieve this, albeit probably not the best way
Touching an ungrounded case equalizes the static potential between yourself, the case and all the components in direct electrical contact with the case. This includes the MB and peripheral card themselves.