one thing i do have to agree with raven on,
Now it may fail on size, noise, heat, price or speed but to the people that will be buying these cards the amount of vram would be a small factor in my opinion.
also to say niche market, don't go there. Why, the bottom line is these are the exclusive end of the market, only a very select few will buy them, and then a even more select few of them will be running with 3 or more monitors, of course only a few of them will play those games in question where the Vram limit might come into play. I think you see the point.
These are not high volume parts, now I'm not suggesting that there will only be 1000 made like some rumours, but these certainly wont be a very high volume like the GTX580/AMD6970.