Rubén spotted a new interface design and posted a YouTube video of it in action. He posted the details on Google Blogoscoped Forums and his blog and describes how you can change your cookies to see this test.
The video shows how the left search filters and the middle search results have their own scroll bars. Here is the video:
Here is a static picture:
This is a pretty interesting test and I am not sure if it will last, but it does seem like Google is testing this internally.
Forum discussion at Google Blogoscoped Forums.
Update: This is now confirmed, Google told me "Google is always experimenting with new features."