Since it's not out yet that's really difficult to say. I think it's going to have a higher quality finish compared to iPods. MacBooks are anodized and don't seem to scratch easily. I have an anodized "Mil-Spec" flashlight that has yet to scratch and it's about eight years old.
I posted this before but here it is one more time. Straight from Apple's iPhone design webpage (side note half way down the page):
http://www.apple.com/iphone/design/
"Aluminum and Glass Body
The back of iPhone 5 is made of anodized 6000 series aluminum the same material used in Apple notebooks with inlays along the top and bottom made of ceramic glass (on the white and silver model) or pigmented glass (on the black and slate model)."
If it's the same quality and thickness of what they use on MacBooks I feel we have little to worry about. I really don't think they'll skimp on a flagship device. Black over silver (aluminum) is risky though, I will say that. If it does scratch through it will be very noticeable. For me it would add character, but I understand that others may not feel the same way.