Google users in the US, UK, Canada, and a handful of other countries will notice that the page has gone black today. Do not adjust your monitors, this is intentional. Google is trying to draw attention to Earth Hour, an initiative to convince people to use less energy.
Now here's the thing, last year there a minor controversy over whether Google should permanently change its background to black in order to save energy. The theory is that it takes less energy to display a black background than a white one. But while that might have been somewhat true a few years ago when most computer users had cathode ray tube monitors, it actually takes more energy to display a black background on some modern LCD displays. So umm, essentially Google's black web page today could be contributing to an increase in energy use.
Hopefully if the move prompts a few folks to think about energy conservation, there'll be a net gain. But umm, seriously, Google, why go out of your way to debunk the myth of a energy-saving black Google last year just to promote it yourself today?