It's actually 1.5 to 2 times screen width for HDTV material displayed on your TV. For DVD you should move back further, and for STANDARD TV (cable/DSS) it is something like 4 times the screen width for optimal viewing.
Obviously, 4 times screen width (16+ feet) is not likely practical or something you want, but you will definitely pay the price for the quality you get as you move closer and closer to a lower quality source material.
It amazes me how many people get 16:9 HD capable televisions, then don't get HD cable or DSS... or off air. You can't truly experience the quality of your HD television until you run true 1080i or 720p material through it. And no, an upconverted DVD is not ever going to be the same as HDTV.