Okay, this made me curious. So I just ran a controlled test in studio.
Available but constant light (Profoto modeling lights in soft boxes, no variable)
A900, 24-70 @ 70/5.6 on a tripod to keep it a constant. RAW.
Walked through all ISOs from 100 through 3200.
Batch sync processed in CS4 Bridge-ACR.
Opened as 16 bit tiffs in CS4, viewed at 100% on screen.
ISO 320 clearly shows more micro detail than 100 which seems to suppress it, (see the crop where a finger print is more clearly visable in the 320 shot).
320 shows a bit more noise than 100 but not a lot more, and in most practical applications would be a non issue. Color looked a bit better at ISO 320.
No surprise, ISO 200 records more nano detail than 100 but not quite as well as the 320, while exhibiting the noise level very close to the 100.
In all but the most huge enlargements I'd agree that 320 is the optimal balance between recording detail with acceptable noise levels when working at the lower ISO end.
The question is how much sharpening with 100 works, and how much selective noise control for 320 works, and in the end which is better? I suspect that if you sharpen 100 to gain back the detail the noise will be enhanced, so you may as well have shot 200 or 320 in the first place.
I would only resort to 100 if trying to control shallow DOF in extraordinarily bright conditions, or to produce a slow shutter speed, but not for IQ.
HIGH ISOs
I detected very little applied difference between ISO 1250 and 1600. There is some, but the noise structure is very similar.
But when compared to ISO 1000 it was clear that 1000 was the best high ISO because it did not exhibit a salt 'n pepper type grain structure apparent in 1250 and 1600.
Plus, ISOs 1250 and above (but not much in 1250) increasingly showed blue blotches in the solid black areas. ISO 1000 showed no signs of this at all.
My working conclusion would be that 1000 is highly usable on this camera if exposed with the usual care needed for any high ISO application. 800 is better, and 640 even better ... with 640 being the optimal balance between detail and noise in mid range ISOs. But 800 is no slouch.
-Marc