We've gotten reports from a number of you about the official YouTube app being updated, though no notification was given in the Market. We've seen the same thing here. If you go to the "My Apps" section of the Android Market, you won't see that an update's available. Go to the app listing directly, and there it is. And according to that listing, devices on Android 3.2 and up have an update waiting, and you can now sign in with your Google account. (Really? You couldn't do this before?) Other bugfixes are listed as well.
A little confusing, to be sure. But we'll take an update over no update any day of the week.