From what I understand is that if you have a 1080p set, you can sit closer than a 720p set, when watching HD material. If you are watching SD content, you should sit further back so you don't see all the imperfections.
From this poll it seems the most popular distance is twice the diameter of the screen. So on a 42" screen the viewing distance would be 7'. I've seen recommendations of 2 to 21/2 times the screen size.
http://www.digitalhome.ca/forum/showthread.php?t=46708
There is a link to a calculator on post #10 but the best idea is to pace off a distance at a store to find what's comfortable for you and what content display you'll be watching. My feeling is bigger is better but I don't want to be so close that the angles are too large.
Maybe having a chair that can move forwards and backwards depending on the content would be ideal.
