The first reviews are coming in on the 5D2
The base range of the original 5D is 100 to 1600 ISO with expansion to 3200. Canon appears to have succeeded very well in matching and surpassing the low noise and superb resolution of the original 5D. In fact the 5D2, with a base range of 100 to 3200, is every bit as good in that range as the 5D in its 100-1600 range. Most will have no real problem shooting in the 50-3200 ISO range without much regard to noise. That range is a justified option in the Auto setting on the 5D2. The 6400 and 12800 ISO options are certainly usable in most circumstances, but the noise increases as you go up from ISO 3200.
Looking at the Canon 5D Mark II for a day hardly provides enough data to reach conclusions regarding the 5D2. However, the initial results are truly impressive. Unlike the exaggerated claims for the 50D, the 5D Mark II appears to perform as Canon claims. Resolution is as good or better at the same ISO as the original 5D, but the range is extended somewhere between one and three stops in a sensor with 65% more pixels.