YouTube added some new options that allow you to filter search results by type. You can restrict the results to videos uploaded by YouTube partners, videos that have annotations or closed captions. There’s also an option to restrict the results to feature length videos, but it doesn’t seem to work yet.
Use the “partner videos” restriction to get official videos, annotations could be used for additional explanations or interactivity, while closed captions allow you to search inside the text of a video. The closed captioning features has been recently added to YouTube, so you won’t find too many subtitled videos.
This feature is already in Google Videos, which includes YouTube in the search results, so you get more results by searching with Google Video.
And my HomeGrown Captioned Video Search which adds TubeCaption and Overstream to the results.
Hurray for Progress in online captioning!