Google Experiments With New Interface for Results Page
There may be two main reasons for rolling this out now. One, Google has always struggled to make people use alternative searches more often. In user testing sessions I've observed a lot of users who have never used Google's image search, groups or Google News. They are often surprised that these services even exist! I believe Google's transition from tabs to plain links two years ago was to encourage use of those links. Likewise, the new interface for results page will lead more people to try out these alternative searches. If you look at full-page screenshot the links in the sidebar are repeated at the bottom of the results.
Second, with more and more people switching from 800x600 to 1024x768 resolution or above, a lot of space on the right side of the page is left unused. The new interface utilises horizontal space by adding a sidebar.
Google has said in the past that they keep experimenting with new features to improve their services. This may be one of those experiments.