For the record, the G70 is pretty bright at 240 ANSI lumens, and I use a 1.3 gain screen.
The G70 I have now is actually not mine and is therefore not permantely mounted. I did an a/b comparison of the G70 to a Plus DLP a few months ago. In order to get both images on my screen (side by side) I had to physically move the G70 closer in order to get the image small enough for the two images to be on the same screen.
Did I have the setup ISF certified? No. Did I have it converged and focused well? Yes. I have seen other set ups with smaller screens too, and I think I know what a proper ISF setup looks like on smaller screens. Of course, having a totally or near totally dark room becomes increasingly important with a larger screen. I also have very low reflective surfaces in my room.
I guess the 8 ft lamberts minimum comes from ISF folks or other knowledgable people, but if you are like me and prefer a larger screen, at least try it and decide for yourself. IMHO size does matter in home theater, and that comes from a guy who's spent most of his life trying to convience half the population that size is not all that important http://www.avsforum.com/ubb/smile.gif