This should really be etched in everyone's brain and even a stick
BUT:
1. Just because a channel is an HD channel, doesn't meant that everything they broadcast is going to be HD.
2. Even an HD broadcast can have non-HD elements inside it.
3. Just being widescreen or letterboxed does not = HD
4. Being HD does not exclusively mean the 16x9 aspect ratio.
I am sure I may have missed some other points, but these should likely head off some angry questions & comments over the next few days or so.