Is there a chance that the future Nvidia graphics cards, like the 8900 series or 9900 series, will be smaller than 9 inches in length? Not have a gigantic power consumption, and not have a heat problem? All of those would be awesome if they could do that in a graphics card, better than the 8800Ultra, and have over 200 shader units.
The thing is that they will always try to maximize the top end, and that means despite efficiencies achieved by process changes they will clock them to their max and staty pretty power consumptive, not much less than a GTS if at all.
As for board size, they could make a smaller board, but with all the things they need to put on them there's not much room on a smaller board, and even the wire traces alone for memory would require extra layers if they were smaller. The technology could change and their might not need as many power regulators, and such, but it's unlikely they will get noticeably smaller than 9" which is a pretty stedfast standard for ATX cases by now.
What you might end up seeing is current G80/R600 performance found in a more mid-range solution that could have less memory bitwidth (but faster memory) and a smaller more efficient chip, but it wouldn't be the GF9900 series, but more like the 9600 series or something that would achieve that.
A bit off topic but yesterday I was inside my son's computer to clean his GPU which due to neglect was getting quite hot (110c). Having not looked at the 6800gt in that computer for so long it seemed so small. I remember bringing that card home and thinking it was so huge.