there´s a small metall thingy on the mobo that ensures contact to the metal shield of the case. if you have it on the mobo then you probably shouldnt run it when you´re not shure if it is grounded...
EDIT: the little metally thingy
goes through a hole in the mobo that is surrounded with a metall contact.
for me the board was useable when you plug a monitor into the onboard video port (at least until the video hardware died) because that grounded the board as well (I´ve done some trace routing and saw that the ground pins of all ports are all connected to the little metall-thingy-connection)