If you read the SMPTE literature, they don't generally discuss screen sizes and viewing distances as raw numbers. Instead they talk about "subtended angles", and the specific SMPTE reccomendation is that the image being viewed must subtend a minimum of 26 degrees of your horizontal field of view, and a maximum of 55 degrees. The reccomendation for vertical subtended angles is a minimum of 15 degrees and a maximum of 35 degrees. With a 16:9 shaped screen, you achieve these reccomended subtended angles while seated at approximately 1X to 3X the width of the screen image.
The THX reccomendations are similar but they also dicsuss "ideal seating positions" which we often call the "sweet spot" at AVS. The THX reccomendations put you between 1.54X and 1.8X the screen width of a 16:9 screen for the "ideal" seating.
Both the SMPTE and THX viewing distance reccomendations have the same objective, to place a viewer in "total immersion" in the video image. This is akin to a theater experience, whereas "watching TV" is a part time experience that takes place at greater viewing distances.
In practical terms the typical Home Theater has a second constraint upon minimum seating distance which is the pixel structure of the digital image, or the scanlines from a CRT projector - if you can actually perceive the image structure, you are too close. Your quality of vision plays into this minimum as well.
As a point of reference I have both a 1024X768 (XGA) front projector and a 1080p monitor in my "home theater", which is also my front room.
I watch the monitor mostly, and I view from a distance of 1.5X screen width, 6-7 feet. That is my minimum only because I like to sit in a recliner with my feet up, I could get closer to a 1080p image without seeing the pixels. On my LCD screen you actually start to lose brightness and contrast on the edges of the screen before you see the pixels, from the increased viewing angle. (The recliner is on castors.)
By contrast I used to view the XGA projector (90" diagonal 4:3 screen) from about 1.8X the screen width (10-11 feet) when alone, or 7-9 feet with my glasses off. When my buddies come over for DVDs, we arrange the room seating in a single arc at 2X the screen width. The limitation in the projector case is the "screendoor" or pixel structure of the image. However in a darkened room the front projector has much more impact with it's larger image, even though that is only slightly better than a 3/4 megapixel image, and often masked down to less than half a megapixel for wider aspect ratio DVDs.
The XGA projector is overdue for upgrade, but I'm waiting for prices to come down before going with 1080p and 16:9.
Edit: Sean, you are correct, and the "Quad HD" displays will have 2X the resolution of 1080p.
Wow, this thread just gets better and better.
I was thinking last night , Where do all the ladies way in on this issue?
IT seems that it is 100% Fellas on this thread that give a toss about this sort of thing.
Why do our male brains care so much about these "Details"?
Why do we obsess so much over these things.
Gosh as long as my missus can se a clear picture and hear the dialogue, she is happy.
Sometimes I wish I could see things from her perspective, I might even just watch the movie for a change