Google already integrates in Universal Search video results, books, news, images, local search results and now subscribed links. The next ingredients could be blogs, products, scholar papers and some Google Base verticals.
Even if Google says it ranks results from specialized search engines along with standard web results, this is more like a figure of speech since Google displays groups of search results from Google News, Image Search and Google Maps, not individual results. More likely, Google determines how relevant some image results are for a query and finds the proper placement. Instead of actually mixing results from many specialized search engines, Google includes some of the previous Oneboxes in a controlled fashion. Google adopted a conservative approach in order to not clutter the search results and because it's difficult to compare completely different entities. How to compare some search results for "Christmas" from Digg with a news about Christmas ornaments?
Ask.com decided to separate the specialized results and display them in a column generally used for advertising (like in this search for Christmas). Now Valleywag has a screenshot of a Google experiment that displays the universal results in a separate column.
Does this mean Universal Search didn't work as expected and Google tries alternative interfaces?
{ Thank you, Trey and Jordan. }