Comparison of shadow (blackframe) noise in the Canon 20D at three different ISO settings for 5 different exposure indices. The upper left is the ISO100 original, the middle one the ISO 400, and the lower right, the ISO 1600. Those three are top-left 100% RAW crops, and the other exposure indices on each row are obtained by division and subtraction from an offset of 128 (the 20D RAW black level. Exposure indices are not to be taken literally; they are relative.

Note: for signal levels far above the shadows of any ISO setting the noise is the same for all exposure indices, differing only in the level of quantization.

Note, Oct 16, 2009: I have discovered since writing the original text that RAW quantization is not an issue in the 20D, and most digital cameras.