so does this happens on the 2010 macbooks or not?
Yes this does happen on the new 2010 Macbooks with the unibody shell. The most common cracks appear on the outside casing by the hinges. I also had a rather large crack from my ethernet input all the way to my keyboard. It does not, to my knowledge, inhibit the functionality of the computer but it is an eye sore. I took mine to the apple store yesterday and they are replacing it at no cost. My mac is NO LONGER UNDER WARRANTY, but they are having numerous complaints about it that they are replacing the casing for anyone with the issue.