The WSJ blogger asked the Energy Star folks who asked the Cadmus Group to do a quick test. They found between 5% and 20% power difference between black and white on CRTs and no difference on LCDs. This doesn't really contradict the first article's numbers. He even confirms the 25% of monitors being CRT number, or rather, he states that 3/4 are LCD.
The real point here is that 750 MW-hours per year is (in more simple units) 8.6 x 10^4 Watts which is the equivalent of roughly 1500 60 Watt light bulbs. So Google switching to black background is the approximate equivalent of turning off 1500 light bulbs worldwide. That's a very, very, very small number compared to the total number of 60 Watt bulbs in use all over the world. Furthermore, nearly 100% of all monitors will eventually (probably very soon) be LCD, rendering those 1500 light bulbs moot. Oh well.
Ok. Time to get back to work.