Here are ISO 400 histograms of noise vs lower exposure for a D200; green channel data of a Kodak IT8 chart underexposed by three stops. These are histograms of the grayscale patches along the bottom of the chart. The growing spike at zero as the mean level is reduced is the aggregate weight of the histogram to the left of zero being lumped into a delta function *at* zero by the clipping of black:
The darkest patch (22) approaches the endpoint, which is roughly a half gaussian of unaffected raw values with positive read noise, plus a delta function incorporating all the pixels whose negative raw value has been set to zero by clipping.
From this camera I was getting about 14 electrons of read noise at ISO 100, somewhat in between John's figure and Roger's.