slawski - 6:25 am on Aug 1, 2010 (gmt 0)
There was an announcement about this on the Official Google Blog on April 27th, 2010.
From the blog post:
We've offered a "Similar" feature on results for a while now as a way to discover new, useful sites, but it hasn't been too visible. Since we've been continuously improving this feature and we think it's really useful, we're now going to start showing these alternative sites more prominently. Starting this week, for queries where similar sites are likely to be helpful, we’ll display a list of "Pages similar" at the bottom of the results page.