The guy at the Genius Bar said it's because of that magnet latch and closing it pretty hard or pressure on top.
Ahem.
Thanks kidwithdimples! And congrats on your fix!
So, someone who works on Macs but isn't a Genius says its purely the magnets, and a Genius says its a combination fo the magnet latches, closing the lid hard, or pressure on top.
To infinitethrill:
No one said that the macbooks don't crack. The debate started with the area around the magnets...that's all the OP mentioned. He didn't mention side cracks or anything else.
Now, I don't have to see the internals to see that the two protrusions on the left and right of the edge of the lid appears to be what's damaging the lid. They are arranged horizontally to the screen, and the cracks are horizontal and appear to be directly underneath the protusions when the macbook lid is closed. In the case of the OP, it appears to be on the right-side (although, I've seen pics of both sides being affected).
What would happen if someone sanded those down or softened the edges of the protusions? What about the people who aren't experiencing issues? I'm not experiencing this issue, yet have the same magnets as the OP, most likely. Why am I not having the same issue? And why is the Genius mentioning that its a combination of three issues instead of it being a fault of the magnets alone?
The pictures are cool but aren't really needed to make an educated diagnosis of the issue.
One day, these issues may affect either my macbook, my wife's (more than likely, as I've seen her slamming her lid several times), or maybe both, over time. As for now, I may not even see the issue until after my Applecare runs out (that's quite awhile without any cracks and with normal use).