well, first lets get some semi real numbers to play with (I like to pipe dream too!)
volume: (12"x12"x12") * (10'x7'x4') = 483,840 cubic inches.
231 cubic inches per gallon = apx [b]2094.54 Gallons[b]
Water weight @ 8.81 lbs/gallon = 18,448 pounds
Live Rock @ 2lbs/gallon = 4,000 +/- pounds
Sand substrate @4" deep = 1,000 +/- pounds
Add to that equipment such as pumps, lights, plumbing etc and your total weight will be more towards 24,000 pounds. And that of course doesn't consider the weight of the tank itself as I don't know how much cement weighs other than, "a lot"
I would certainly suggest steel reinforced cement. Unless you plan on using all stainless steel, any other metal strong enough to hold the forces will corrode and fail. Hmm, I wonder about a carbon fiber frame? would have to find some stats on that. I would also suggest 1" acrylic. Not just for the weight of the water in the tank, but as "insurance" against an external force knocking into the tank and causing a failure.
here's somplace that has what i'd like to see in my dream home: http://www.aqds.com/PvtOff.htm