YouTube lets you download the video captions file if you know the URL of the video: http://video.google.com/timedtext?lang=en&v=VIDEO_ID. For e.g. for this video, this's the captions file. It's an XML file and you can extract plain text by removing all the tags. Right-click on the page, select "view source", copy all the text and paste it on this page.
Now it's easy to copy an excerpt from the keynote: