I'm not sure if it's so much the weight as it is the design of heatsinks. The 7700 works because all the weight is right at the motherboard. Same with the Ninja, most of it's weight is at the base.
But look at a tower sink like Coolermaster's Hyper6, I've seen people on other forums cautioning about it being too top heavy. Coolermaster had a good design with that sink, but I think got a bit overzealous doing everything in copper. Scythe recently tried to redo a couple of their heatsinks in all copper, but for the increase in weight the performance wasn't that good. And the added weight wasn't welcomed either. So they were scrapped.
What I would personally like to see happen though in the ATX standard is cases accommodating heatsinks with a reinforced plate and standoffs. Intel has had this requirement for their server platforms for some time now. Definitely wouldn't have to worry about hsf weight when it's bolted to the motherboard tray.