Mike Arrington says that Yahoo has started integrating shared bookmarks from Delicious (which it now owns) into its search results, which is an interesting move — you can see an example here. Underneath each search result, it tells you how many people saved that page as a bookmark in their Delicious account. It doesn’t seem to be affecting the actual ranking of results, but it might do so in the future.
As a commenter pointed out, this is similar to what you see if you have the Stumbleupon toolbar installed. It seems like a smart move for Yahoo to make, not only because it takes advantage of one of the company’s Web 2.0 acquisitions (something Yahoo hasn’t done much of) but also because it adds something extra. And as this commenter at TechCrunch notes, the delicious “votes” on a link can add a fair amount of value, depending on the topic of your search.
Of course, as ParisLemon points out, it would be fairly trivial for Google to do the same thing with all the links that have been shared through Google Reader — something a couple of startups, including Readburner.com, have already begun aggregating. This seems like a natural step to me, and one way of adding the social element to search and competing with Mahalo and other “social search” services.