I so love these post about weight, and the myths, misconceptions that follow them.
1 Weight is always a factor no matter what floor your on, (excluding the concrete slab the house is built on). Water /gal equates to 8.33 lbs, factor in aquarium, stand, substrate, decorations, accessories etc and your talking about a lot.
2 The comparison between the "i weigh 200lbs and my friends weigh 600lbs" stuff is ridiculous, one you and your friends are not sitting in the same spot of the course of time. NO your tank isnt going to go crashing thru the floor as soon as you set it up. (unless your house is like 200 yrs old) what will happen, is (if not at a load bearing wall) the beams underneath will begin to sag over time. Then eventually you may have a beam crack or even break.
3, Weight distribution is what everyone needs to think about and what no one ever mentions. Those iron stands put all of the weight at 4 tiny points. this is what will cause problems in the LONG TERM. a stand that evenly distributes the weight along the course of runners both thru the width and depth of the tank is your best deal. AND yes use an outseide wall running accross the support beams(if you can find how they run) for best support. Something in the middle of the room in excess lbs just screams disaster.
4, OH the other comparison about bathrooms /waterbeds. One, a bathroom is constructed with a sole purpose. xtra support is thrown into these rooms because obviously a cast iron tub, or porcelin, and water is gonna be heavy. BUT even still, this is not weight that is there constantly.
waterbeds, Just use your head, look at the base of a waterbed the surface area it takes up. the wieght is distributed through a huge surface area and therefore no one are has a heavy load on it.
Hope this helps and happy fish keeping
Kevin