MSDN tagging is live. You can see the most popular tags on the MSDN library home page. Also check out the entire tag cloud.
The tag control shows up on all documentation that has opted in to the Community Content feature (currently 1.5 million pages across 7 languages). There is a page-level tagging control just below the topic and one added to each Community Content block. It looks like this:
You can use the Recent Edits RSS to find topics for tagging. And Molly Bostic showed me another cool feature: you can subscribe to an RSS feed for any tag or combination of tags. For example, the following feed shows topics and community content tagged with C# and code: http://msdn2.microsoft.com/en-us/library/Tags-Cloud(d=rss).aspx?tag=c%23+code.
MSDN Community Content with tags.