After nearly three years of on-and-off work, my Halo Ringworld is complete and has been on display at BrickCon 2012. I can't even begin to tell you how relieved I am to finally be done with this epic project! I was honored that it was chosen to win "Best Large Planetary Structure" in the Space theme.
The outer ring (hull) consists of eight more or less identical segments (with minor variations in texture or the use of old gray for contrast), each 72 studs long. The inner ring (landscape) floats freely within the outer ring and stays in place purely by friction. It took over two hours (with help!) to set up, and there's no way I'll be able to do it at home. However, this is most definitely being kept safe and will return at future shows.
My best estimate of its parts count is somewhere around 11,000. The outer ring is fairly consistent in its construction and consists of about 7773 parts, give or take a few. The inner ring, though, is extremely varied--if I had to guess, I'd say it averages around 6 or 7 parts per each of its ~570 studs of circumference.