We have a code to embed YouTube video within a post. It does not impact our bandwidth at all. It's the same as if you clicked the YouTube link, except it doesn't open in a new window, it runs right inside your post. Also, the video comes up paused, so no one's slow connection is saturated unexpectedly. If you are blocking flash-based ads, the video will just show as a blank block with the flash logo in it, which you can press to view.
Use the movie code from the YouTube URL in your browser. For example, http://youtube.com/watch?v=JFwCCL0Vh6U (Movie code highlighted in red.)
Clicking on the YouTube icon above the post editing window will insert the [YOUTUBE] BBCode tags. Simply paste the movie code described above between them, and that's all there is to it.