Erick Schonfeld writes at TechCrunch. Schonefeld's guess is that it may be a partnership with Hulu, but the folks over at Mahalo have another interesting theory. They discovered a new link below a YouTube player today that flips playback over to a higher-quality copy of the video on this page. You can visit that page, for an Amy Sedaris interview with David Letterman, or see the screencapture from Mahalo, embedded below.
Update: YouTube Expands API Set - Including Upload, Player APIs (this was the actual announcement tonight, but perhaps high quality video is still to come)