You'll see a new Youtube button on the toolbar. It lets you embed a video right into the post itself.
This is how it works:
You go to a youtube video, for example, http://youtube.com/watch?v=j96uqRKzHDI
and then copy that code at the end, after the "v=".
Then click on the youtube button in the toolbar and paste in that code.
Then you get:
Cool?.
This only works for Youtube videos, by the way.