I haven't got into this yet, but I think it may be one of my next targets. Take lower quality content for example and upsample it using a Lanzcos 4 engine for example. I am fluent in Scenarist, but I am not sure what the latest tools are for HD content. Anyway, here is a blog I see that deals with the subject incase anyone is also interested in learning how to Author HD DVD:
http://blogs.msdn.com/ptorr/default.aspx
-S