Right now, the Xbox Live page has been updated and the demo has been launched! The only problem is the demo is actually a video of someone playing the demo.
No, we're not kidding. Yes we realize that putting a banner claiming a Ninja Gaiden II Demo was forthcoming and posting a video is a little misleading. So, the demo is up for you to try. If you try and watch it that is! Something tells me this will not go over well with the fans.