I always try to follow the Lechner distance. That is the optimum distance for a given resolution and screen size where you can't see the pixels and appear blocky yet at the same time you don't lose detail.
It works out for 1080p it is best to be a distance from the screen of about 1.6x the diagonal screen diameter. That means for a 70" 1080p screen it is best to sit about 9 feet away
But for 720p it is best to view from a distance about 2.4x the diagonal screen diameter so about a 46 inch screen is really need for sitting at 9 feet.
I also find I naturally sit at about 9 feet from the TV screen as it is the distance that leaves the best viewing angle and elbow room for other people.
I'm staying at my parents' this summer where they have a 40 inch HDTV sitting from 12 feet works for SDTV broadcasts and DVD movies. But I don't see the point in investing in blu-rays, I have played some via my PS3 and with the size of my parents' HDTV and how the living room is arrange where I can't get any closer, at that distance I can't tell the difference, or at least not 3x the price kinda difference.
I normally sit quite close to my computer monitor as some sites (like this) have strangely small font size... Not a problem at typical distances for laptop (screen fixed to keyboard and arms only so long) but does restrict a comfortable posture when web browsing.
