The technology that Google utilises is getting more and more powerful. The Google Speech Team has setup a system whereby the videos that appear in the Politicans Channels are automatically transcribed and indexed as text.
This is focused on the US election but imagine the possibilites of this functionality are huge. Youtube videos will not need tags to be searchable (tags will still help of course).
Not only are you able to search on the text used in the speeches, but if you search for specific text you can automatically jump to where that text is used.
More info can be found on the Google BlogTweet