[video=youtube;56rBrG1MAaQ]http://www.youtube.com/watch?v=56rBrG1MAaQ[/video]
^ example. There is two different BBcodes for youtube one.
[VIDEO]http://www.youtube.com/watch?v=56rBrG1MAaQ[/VIDEO]
[youtube]56rBrG1MAaQ[/youtube]
But this could be considered against the rules, as it's bypassing the one per-video post thingo.
EDIT: Mods, don't infract me for this post, I was merely pointing out how to do, and one of the codes should be removed.