NO NO No.... don't allow HTML - that's a disaster waiting to happen. There is a youtube BBCode available that will allow you to post the youtube number between tags.
Drammy (or another admin) : PM me... I have the code... happy to let you have it...
This is what it YouTube videos look like on forums when using it...
http://www.themouthpiece.com/vb/showthread.php?t=26760