D100 noise reduction mode, how does it work?

Anyone know?

The D100 samples the dark current noise and subtracts it from the image. For a better explanation, search www.nikonians.net or www.dpreview.com.

You can't just subtract noise since it's random!

A digital sensor detects light by converting incident photons into electronic charge, but a small amount of charge that's generated by heat is also captured by the sensor. This small signal increases as a function of the exposure time, and can be significantly large during long exposures. The noise reduction mode in the D100 is a common technique used in digital imaging where an extra frame is captured with the sensor covered, at the same (or close to) exposure time as the image frame, then this 'dark' frame is subtracted from the image frame to remove the dark noise signal. It works well because the thermal noise generation rate does not vary if the temperature does not change.

So it is just a subtraction of the DC component of the noise. What about the time-varying component of the noise?

As the name implies, the noise will be reduced but not eliminated. There are fluctuations in the dark current that's generated at each pixel that will result in a fixed pattern noise after dark frame subtraction, and the statistical shot noise due to the dark current cannot be removed. This is why cooled CCDs are used in scientific instruments to reduce the dark current levels to negligible values.