Back in July, there were reports that Google launched a service which allows YouTube users to mark videos with text labels indicating what certain characters are saying in a segment of the video.
At that time, this service was released as a widget for iGoogle (Google's personalized Google homepage). Today, Google Labs has officially released it as a dedicated tag page.
On this page, you can search for keywords that appear in videos, and these instances are marked with yellow highlights on the timeline.
Currently, the service only supports English searches and is limited to videos on the YouTube website.
In the FAQ document about this service, Google mentioned that this is just the first step, and they hope to eventually expand it to more regions, languages, and videos from other websites.