Well, thse black covers I remmber were avaialble in hongkong for current phones starting from around 2$. They repalced the entire aluminum cover of the iphone back.
The white plastic backs were also avialble covers from hongkong, I remember I saw them a while back on eBay.. lol
Either the iPhone 3G will not look diffrent OR it will look real diffrent (maybe more like the touch?) who knows, we will at least know exactly in a few days.