A very interesting thread has been going on and on for the last 2 weeks on dpreview. One of the members, John Sheehy, posted the following graph showing the Canon EOS 30D Blackframe Noise.
The absolute noise is the noise you get when you use your Canon EOS 30D with en Exposure Compensation of 0, while relative noise is when you shoot to the right without clipping. Relative noise is only relevant if you shoot RAW
This graph clearly shows that the lowest noise of the Canon EOS 30D is given at ISO 160 and not at ISO 100 (looking at the absolute noise curve), and that ISO 160, 320, 640, 1280 are clearly better than their 125, 250, 500, 1000 counterparts.
We can also notice that oddly enough ISO 125 gives black with as much noise as ISO 640 on the Canon EOS 30D with the same Exposure Compensation.
John also posted the following table showing the blackframe noise for both the Canon EOS 10D and the Canon EOS 30D
But John went even further and tested some RAW files of the Canon EOS 5D and it seems that, for the Canon EOS 5D the ISO 100,200,400,800,1600 group is the best in terms of noise performance (compared to the ISO 160,320,640,1280 group in the Canon EOS 30D) and that the ISO 160,320,640,1280 group is the worse (compared to the ISO 125, 250, 500, 1000 on the Canon EOS 30D). Blackframe noise results for the Canon EOS 5D can be found in the following table
Finally as a conclusion, if you are a JPG shooter or RAW shooter that is used to keep you exposure compensation set to 0, it is best for you to shoot at ISOs 160, 320, 640, 1280 (with the Canon EOS 30D).
But if you are exposing to the right for RAW, you can gain the benefits of the Canon EOS 30D 160, 320, 640, 1280 group (in terms of noise) while using the ISO 100, 200, 400, 800, 1600 group, without the slight posterization.
If you feel adventurous enough to go through a 94 posts thread you can find it