See in "Original" to see the noise pattern clearly.
Noise at low iso setting is a main concern for me as I mainly do landscape photography. Problems with noise normally only becomes a problem when making bw pictures. Here the problem can be minimized by controlling the colour noise.
Anyway here is a test with exposure times from 0.4 to 5 sec paired with ISO values from 640 down to 50 respectively. The RAW files where converted by Camera Raw 6.3 with all noise reduction disabled. The exposure of the images were raised 0.75 EV. Only the blue channel is shown as here the most noise is revealed.
Now the 50 ISO is by far the most noise free. But the interesting point is ISO 160 show less noise than ISO 100 and ISO 125 show more noise. The reason for this behaviour is as follows: Going from 100 to 200, 400 ... the amplifier gain before the analogue/digital conversion is increased. However, shooting at ISO 50 the camera is using the same gain as ISO 100 but you are just increasing the exposure time and thus reducing the noise. The problem is more clipping in the highlights. Shooting at ISO 125 the camera is again using the same gain as at ISO 100 but the shutter time is decreased. Shotting at ISO 160 the camera is using the gain of ISO 200.
Shooting at ISO 50, 160, 320 and 640 give low noise but also some clipping of highlights. ISO 125, 250 and 500 gives the most noisy images. Shooting at ISO 100, 200 and 400 might be the best compromise of low noise and avoiding clipping of highlights. However I have started using ISO 50 as my normal setting and then I use multiple exposures to ensure that no highlight is clipped and finally if necessary in Photoshop combine different exposures. The difference in clipping is not so severe between ISO 50 and ISO 100 in my opinion.
This is also reported several places on the web.
See http://dokumentaryfotografr.blogspot.com/2010/02/canon-5d-mark-ii-iso-settings-and-noise.html http://photography-on-the.net/forum/showthread.php?t=922906