You'll maybe notice the new youtube specific video embedding button in the wysiwyg editor on here - so you can now post your videos. Pressing the button inserts the tags required to allow the video to display in the post - but you need to add your video code from youtube between these two tags.
The video code from youtube is just the last part from the URL for that video.
so for example
http://www.youtube.com/watch?v=629NYc_mb_M
You'd take the last part AFTER the '=' sign
629NYc_mb_M and paste that between the youTube tags. That's it!
The code in the editor will end up looking a little like this ('O' replaced with zero in this example so it will show up rather than display the video)
[Y0UTUBE]629NYc_mb_M[/Y0UTUBE]