The basic issue here I think is, to what extent is resolution compromised at high ISO? For example, although the Canon 5D has exlempary noise characteristrics at ISO 3200, there is a slight blurring of resolution, at ISO 1600 too.
I recall the 1Ds2 did not suffer as much from such smearing, but as a consequence appeared slightly noisier than the 5D at maximum ISO.
The D2X of course lost a lot of resolution at high ISO with noise reduction on whereas the new Canon 40D appears to have a noise reduction feature which does not degrade resolution at all, compared with noise reduction off.
Those fortunate enough to own a copy of CS3E will be aware that stacked multiple images can be auto-aligned so well that it's possible to reduce noise significantly, by 2 stops or so, from a hand-held burst of 5 or more shots. Not much good if the subject is moving of course, but for still life in low light it seems to work.
The 5D has a relatively slow continuous frame rate, yet I've been able to take a burst of 5 shots at 1/13th sec at ISO 3200 (using IS) and auto-align and stack them so well that the result looks sharper than any single shot from the series; sharper no doubt only as a result of lower noise, equal to that from an ISO 800 setting, but probably not as sharp as stacking 5 images of the same scene shot at ISO 800.