As for talk of PentiumM architectures, their design brief is battery life, first and foremost. Although good for general computing, these chips aren't good for say video editing.
The Althon 64 X2 processors changed the Althon's performance in video encoding when compared to the Pentium 4 due to their high efficiency. Given that the Pentium-M has even higher efficiency, two of them (Yonah) should be able to roughly match or outperform a high end Pentium 4 in video encoding.
As for nVidia's lower wattage graphic cards, most of this is down to the shrink of the GPU's die size (reduced from 130nm to 110nm). Again it's a happy coincidence that the resultant chips run a little cooler. If they don't have a further die shrink, their next generation cards will run hotter. Will we eventually see external power bricks for 3d cards, thus showing a little mercy to our PSUs?
Most of it is actually the result of the transistors they used for the 110nm process. I don't know how TSMC and Nvidia collaborate with the designs but if there are mutiple transistors avaliable (as is true for Intel and IBM's processor design teams), then they probably selected the ones that provided the best performance while keeping the thermal envelope smaller than the GeForce 6800 Ultra's.
When I read stories that 800W psu's are being developed, even if we assume 80% efficiency, that's potentially 160W of heat having to be dissipated from the PSU alone. Of course thats assuming that a PSU is running at 100% load. Of course that sounds ridiculous, TODAY, but when you consider how much more your pc is doing, the baseline for minimum wattage has been on the increase. A few years ago, 150W was the average, now it's around the 250W mark. As the average load goes up so does the amount of heat generated, and we've only talked about the PSU, what about the heat emitted from the CPU, VGA card, mobo, hard-drives, (there's even going to be add-on card which will compute real-world physics, expect this to be the next big thing, just as 3d cards were).
It is actually 200 Watts of power wasted if you take into account that the 800 Watt figure is the maximum DC power provided rather than AC power used.
Also, my tower uses less than 147 Watts of AC Power (I'm going by my UPS's measurement, which has other things in the figure) so 250 Watts of AC Power is far from average. Especially if we're talking about DC power rather than AC power.