To recap, Universal Search is a term coined by Google in which results from different vertical search engines (images, video, news etc...) are integrated into Google's regular search results.  The idea is that textual listings may not necessarily be the most relevant result for a user's query.

But when will these different search results show up? For example, what causes a video of a scene from Star Wars to appear within the organic search results for the query: "Darth Vader." Obviously it has to do with Google's sophisticated algorithms which can correlate the right content to a user's query, however here are some factors that I think can help influence the Universal Search results:

1. Number of Searches per Individual Search Silo - This theory is based on shear volume for a given query on a given vertical search engine. For instance if the query "scenic mountains" has a high number of searches within the image search, perhaps this influences the top images to appear within Universal search for the same query.  
2. Performance through ratings and views - This applies mostly to Video Search because on YouTube and Google Video both ratings and the number of views are kept track of. Could the ratings/views affect the appearance of videos with Universal Search?
3. Clicks per listing - Google applies this same type of algorithm on both AdWords and organic search results therefore it's something that can help Google gauge the relevance of non-text based results.
4. Time Spent - With the Google Toolbar installed on so many users' computers as well Google Analytics installed on so many websites, Google can start to understand how engaged users are with a website (after a user clicked on a listing) by measuring time spent.
5. Return Visits - If a user clicks on a result listing, visits a website/image/news listing, returns immediately and clicks on another listing and continues this cycle, will Google start decreasing the quality of the websites/image/news listings that were selected?
6. Usability - While at Searchology I found out that Google performs rigorous usability testing using eye tracking to further understand users' search behavior and reactions to different types of search results.

How Can You Leverage Universal Search?

Universal Search requires a wide-range content strategy in addition to simply text.  Here are some tactics worth looking into in terms of Universal Search:

• When using images in your web pages consider adding related keywords to both the name of the image (filename - ie: imagename.jpg) and the alt text
• When uploading videos to YouTube and Google video take full advantage of adding a full description about your video.
• Promote your uploaded videos on your blog or through press releases.  The higher your ratings and the number of views, the more authoritative your video.
• Apply to be a Google News Source. If this doesn't work, submit your latest content to sites which are Google News Sources.
• Keep visitors on your site longer by adding enticing content which compares competitors, offers pricing and gives them something tangible to take away (whitepapers, data sheets).