It seems there’s some truth in this quote from Futurama’s Fry.
According to this article the human eye has a resolution of 1,6 arc minute for a line pair. That is, two lines at a distance can be seen as distinct lines when the angle (between the two rays that go from the eye to each line) is not smaller than 1,6 arc minute.
Let’s say that those two lines correspond to 2 pixels (I’d have to say 3, black white black, but the article takes 2). That would mean that 1 eye-pixel is about 0,8 arc minute or 0,013 degrees.
Now take your average HDTV, 30” diagonal, 1366x768 resolution. Using the Pythagorean theorem this works out to a pixel size of about 0,5mm.
What would be the ideal distance at which to put this TV?
Now using some trigonometry, Tan(angle) = opposite/adjacent. opposite is our pixel, 0,5mm; adjacent is the distance between us and the TV, which is what we want to know.
Conclusion: do not put your HDTV at a distance of more than 2,2m!