We're now moving that out of beta-testing and calling it "official" functionality. At the same time, we're also opening up the feature for use by Upgrade Members and above!
Here's a copy and paste of some info from the original post:
Some Notes and Instructions: The new WritingML embed tag can only be used within Book Items (blogs, etc). It will not work anywhere else within Writing.Com.
The tag takes 2 arguments, a "source type" and a URL.
* The only "source type" presently available is youtube.
* The URL is just a copy and paste of the URL that you provides under the video's info. It should look like this: http://www.youtube.com/watch?v=zlfKdbWwruY Only one embed tag may be used per entry.
An Example: {center}{embed:youtube:http://www.youtube.com/watch?v=zlfKdbWwruY}{/center}
Given that "youtube" is the only source type, the tag will basically look like this:
{embed:youtube:URL}
Now, for your creative entertainment, the above tag yields the following:
Please let us know about any issues, suggestions, etc...
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At the same time, we're also opening up the feature for use by Upgrade Members and above!
Given that "youtube" is the only source type, the tag will basically look like this: 
