Anything I watch on Science HD is in HD. I love that channel. Admittedly I usually watch Really Big Things, Master Blasters, How It's Made, How Do They Do It, and Build It Bigger.
Anything I've watched on Nat Geo is in HD, but I only go over there when I happen to notice somethings on. I've seen some really good shows about the Tenerife disaster, and about the construction of the Airbus A380. I just never think to go over there.
Needless to say, Science HD makes my HD plan worth it in and of itself. It was the first channel I started watching when I upgraded to my 722. They had a How It's Made marathon on..