I have done a bit of a small rework in the rich-text editor and it's now possible to post YouTube videos in our forums.
For this to work you must be using the rich-text editor - it won't work in the Quick Reply box and will get break your code. Also, like the Image button you have to enter the URL for the video, then press TAB to move away from the field. This will update the size. Then you can click the Update button.