rad said:
It is true that there is a correlation between the ability to resolve detail and the distance to an image. It will vary by person, but most people will usually be within a fairly narrow window.
One of the reasons why manufacturers are not making some of the 1080p TVs at below 56", and I don't believe any rear projection 1080p is below 50", is that it is hard to see a difference between 720p and 1080p at smaller sizes and typical viewing distances.
That said, even if it is true that most people do not any more than 720p from a 50" screen at 9', a lot of people sit closer than 9' to a 50" screen. Also, people tend to sit closer to 1080p sets because you can't see the pixel structure and thus it enables you to sit close without that annoying artifact. And many HD sets being sold today are 55" and larger.
Thus this bit of information does not provide a good excuse for an HD program/channel to be broadcast in 720p. Instead of using a 50" image at a viewing distance of 9' as a standard, it should be more along the lines of a 60" image at 8' (some people exceed this). And since most people can appreciate the increased resolution of a 1080p image at 60" & 8', then it is reasonable to expect HD to be delivered at the higher resolution.
