It has happened. After 2 solid years of service, my MacBook has cracked on the top case, in the above picture.
I thought I was being careful with my computer, but even with careful usage, it seems that a combination of the monitor spacers and palm pressure cracks the case on the edge. Naturally the new unibody MacBooks don’t show this problem.
If this problem has happened to your computer, Apple now repairs the case free of charge, regardless of warranty. Take it to the Genius Bar and it should be taken care of.