Always works for me.
I copy the URL from the Browser.
IE
http://www.youtube.com/watch?v=ne1DUzHCoA4
Then return to my post IE Here..
Click the Youtube button and you get
[youtube][/youtube]
Then I put my cursor between the two inner brackets and click paste..
IE here [youtube
][/youtube]
[youtube]
[/youtube]
and you get this...
[youtube]http://www.youtube.com/watch?v=ne1DUzHCoA4[/youtube]
Works every time...