From the Official Google Blog:

We’ve known it for a long time: the web is big. The first Google index in 1998 already had 26 million pages, and by 2000 the Google index reached the one billion mark. Over the last eight years, we’ve seen a lot of big numbers about how much content is really out there. Recently, even our search engineers stopped in awe about just how big the web is these days — when our systems that process links on the web to find new content hit a milestone: 1 trillion (as in 1,000,000,000,000) unique URLs on the web at once!