Turns out "later in December" wasn't that long at all for YouTube. The video portal is now available as an Xbox Live app for beta users, rolling out less than one week after the new Dashboard. It should appear in the apps marketplace window. YouTube shares the same Metro UI look as the rest of the platform and features Kinect support as well as access to your Watch Later, Playlists, Favorites, and Subscriptions. You can like / dislike from within the video, show captions and get a glimpse of the description. No sign of comments anywhere, but many will regard this as a Good Thing. HD playback works clearly but not without a few infinite hiccups that required backing in and out, and no matter how hard we tried, we couldn't get our favorite Phoenix and Daft Punk bootleg concert video to work. Here's hoping public release isn't too far off.
[Thanks to everyone who sent this in!]