For the moment you can use this example code :

<iframe width="560" height="315" src="http://www.youtube.com/embed/video_id" frameborder="0"
allowfullscreen></iframe>

Where "video_id" is the ID of the youtube.com hosted video that you'd like to embed.

See also here :
http://wordpress.org/support/topic/embedding-youtube-videos-into-an-event?replies=9
http://wordpress.org/support/topic/auto-embed-movies-from-youtubevimeo-the-right-way