In the case of CRT monitors, this is true, but with the prevalence of LCD displays these days, it's nonsense. LCD displays are backlit with a fluorescent tube which is on, at the same intensity, all the time. A black pixel on an LCD display completely obscures the light coming out of the tube.
Well, you hope it does - that's why LCD makers brag about how black their blacks are - a really good black mask is actually pretty difficult to attain, so most LCDs show black as a sort of vaguely glow-ey grey. Which is why you have to power your monitor off if it's in your bedroom - a completely black screen is still bright enough to keep you awake. And anyway, that actually does save power.
But the idea that all of the web designers in the world should drop all of their aesthetic efforts and switch to pure white-on-black or some other cockamamie idea is going to be a real bummer if it catches on. I've grown to appreciate visual design over the years. I hate to say it, but it's one of the reasons I can't stomach using a Linux box or a Windows box - the design is just too visually unpleasant now that I've been spoiled by Mac OS X. Thanks, Steve. :')