The new .NET SDK is released and available for download here:
There’s now updated support for YouTube V2 and a new vertical object model that allows you to use local LINQ queries. Please go through the Google.YouTube namespaces and see what’s new there.
Documentation for it here: http://google-gdata.googlecode.com/svn/trunk/clients/cs/docs/AdditionalContent/YouTubeLinqExamples.html
And there is the Notifier for YouTube sample application, which showcases the activity feeds YouTube is exposing. You can subscribe to events from your friends and other YouTube users and get notified whenever they leave their marks in the YouTube universe. The sample is also available as a separate download