Ah! I remember reading about this for cleaning the D1 CCD - it is misleading. I'm not sure I share Nikon's faith in uninterrupted AC power supply to keep the mirror up while you're doing CCD hygiene, but I suppose they've calculated the odds. I'd plug the AC adapter into my computer's UPS just in case Murphy's Law came into play.
I wonder what the tradeoff is with the "antishock" mirror custom setting. Slower "motor drive" speed?