http://www.trustedreviews.com/articl...ge=7573&head=0
"...Another important point is that a television MUST conform to a widescreen aspect ratio in order to sport the HD Ready logo, which could be the final death knell for 4:3 TVs..."
Basically, to be "true" HD, you need to be in a wide aspect ratio.
I think things are going to be a bit fuzzy on the subject for quite a while, but the standard says wide (16:9).
Read the whole article, it is all you would ever want to know about HD... very well done.