Dug back through some older pictures. The 5D outperformed my 40D in terms of noise, by a good amount at ISO 3200. The difference when underexposed by 1 stop and brought back up in post-processing was even more pronounced.
To be fair, I'm not sure that the 40D was quite as good as the 30D in terms of high-ISO noise. I never had the two at the same time to compare, though I think that the 30D had a little more latitude at higher ISO settings. Going by memory here so I can't really say for certain.
The 5D2 is better than the original 5D, but I don't think it's the proverbial 1-stop difference that many have cited. I'm thinking more like 2/3 of a stop. I think I posted something on this back in February or March when I first got the 5D2 but I can't remember if it was here or on another forum. I've got some old test shots, but I haven't converted to JPG or posted them on the net.
Two points on that comparison - in order to get the same exposure at the same ISO (3200 in this case) and f/stop, I had to shoot the 5D at 1/3 of a stop faster shutter. Not really an advantage, but I did take some care to equalize the exposure of the two cameras. The other point - while the 5D2 has slightly better ISO performance when comparing 100% crops between the two cameras (perhaps 1/3 stop better), when the image sizes are equalized (downsizing the 5D2 to match the pixel dimensions of the 5D), the difference is more pronounced.