[youtube]http://www.youtube.com/watch?v=GKzP_9U6vYM[/youtube]
Doesn't work!
You have to have the plugin installed for it, it's not automatic. Search around in the mods section here, or buy the plugin via subscription at
http://mybbcentral.com 
You can use the
AutoMedia plugin for that, you don't need to use [youtube] tags then.
Or there's many YouTube Mycodes like
this.
Regular Expression
\[youtube\]http://www.youtube.com/watch\?v=(.*)\[/youtube\]
Replacement
<div style="margin:0;padding:0;width:642px;"><div style="float:right;"><button onclick="MyBB.popupWindow('http://www.youtube.com/watch_popup?v=$1','youtube_$1',640,480);" style="background:url(http://s.ytimg.com/yt/img/master-vfl102488.png) 0 -658px;border:0;height:22px;width:27px;" /></div><object type="application/x-shockwave-flash" data="http://www.youtube.com/v/$1&fs=1&hd=1" width="560" height="378"><param name="movie" value="http://www.youtube.com/v/$1&fs=1&hd=1" /><param name="allowfullscreen" value="true" /><param name="allowscriptaccess" value="always" /><param name="wmode" value="transparent" /></object></div>
MyBB 1.6 has video tags built in, for now you will have to use a plugin or MyCode
