I found out that KM added a metallic shield for the CCD in the 5D. So, that may be part of the reason we're seeing good high ISO performance from it.
A poster in another forum pointed it out, noting that some owners of both models think the 5D has the advantage at higher ISO speeds from a noise perspective.
Here is a slide show from KM in France.
Slide 7 mentions the new shield.
Konica Minolta France Slide Show Comparison of 5D and 7D