I currently have the 40D and the 5D Mark II. I've found the noise on the 5D Mark II isn't bad at all. I've shot anywhere from 4000, 5000, and 6400 ISO. I mean, you'll def. show some noise on darker clothing, and darker areas, but I've been very impressed so far. Others might have more to add or may not be as happy.
I try and keep the ISO as low as I can in each situation, but I'm completely confident when it comes to a higher ISO with this body.