>Don't you wonder what the low-light capability would be if >Nikon made a ~6-MP camera using current sensor tech? >
It would be no better than what we get now, as long as you downsize the output to 6mpx.
There are only two sources of noise, Schott (or Shot) noise and read noise...
Schott noise is caused by the fundamental properties of light and is the major source of noise. Downsizing an image perfectly emulates a smaller sensor, in terms of Schott noise.
Read noise is caused by imperfect electronics. It is possible that fewer pixels might result in less read noise (per pixel) but I don't think there is any real world evidence of that in the current camera offerings. And consider that downsizing averages out read noise too, so even if the higher density sensor delivers more noise per pixel the aggregate (downsized to 6mpx) would not necessarily be worse.
For example, the D800 and D4 have link:http://home.comcast.net/~NikonD70/Charts/PDR.htm#D800,D4|very similar noise performance] when measured based on a standard output size. The D4 is ever slightly better than the D800 at high ISO's - but very slightly. The D800 is better at or near the base ISO where argualy noise doesn't really matter as much).