Important!: MyBB has been updated, you can (and should) now post youtube videos like so:
[.video=youtube.]http://www.youtube.com/watch?v=k43fKYc6Qos[./.video.] (without the . s!). You can use it for other video sharing sites by changing it like this:
example, vimeo: [.video=vimeo.]http://vimeo.com/12732416[./.video.]