There seems to be a new problem with the white Macbooks reacting to the sweat in their owner's skin: See: http://discussions.apple.com/message.jspa?message
I D=2819568 and (self link) http://toxicsoftware.com/blog/macbook-tarnishing/ . I have some photos on Flickr showing the damage. So far I've only found one other case like this - but i imagine there will be more. Whatever material Apple is/was using on the Macbooks they really need to test it better.