[dionfan]

It's because they're promoting Earth Hour.

Google users in Canada will notice today that we "turned the lights out" on the Google.ca homepage as a gesture to raise awareness of a worldwide energy conservation effort called Earth Hour. As to why we don't do this permanently - it saves no energy; modern displays use the same amount of power regardless of what they display. However, you can do something to reduce the energy consumption of your home PC by joining the Climate Savers Computing Initiative.
On Saturday, March 29, 2008, Earth Hour invites people around the world to turn off their lights for one hour – from 8:00pm to 9:00pm in their local time zone. On this day, cities around the world, including Copenhagen, Chicago, Melbourne, Dubai, and Tel Aviv, will hold events to acknowledge their commitment to energy conservation.

http://www.google.ca/intl/en/earthhour/