When on the Hulu Labs page, scroll down to find the actual projects in Labs.
Along with caption search, Hulu is releasing "heat maps." This graphs user interest along a timeline of a given video that has caption search.
You can find the heat map under the "Captions" tab, located under the video you're watching. Hover over a bar on the heat map to see where it corresponds in the video.
There's no doubt this is an attempt to compete with YouTube, which recently launched Automatic Captions which it hopes will help with search. But Hulu comes out ahead of YouTube on day one, likely because of the ease of getting transcriptions of shows featured on the site.
YouTube also offers a feature similar to heat maps in its analytics feature known as YouTube Insight.
It's nice to see this kind of innovation from a competitor. While YouTube dominates online video, Hulu has made strides in the niche since launch in 2007. Not only does that have implications for web user behavior, but it's also been an eye opener for the television industry as well.