I’ve had my Macbook for 6 months. I saved a lot of money because it was the last of the first generation available and it was a floor model.
I’d like to say I haven’t had any problems with my MacBook, but in June I had to replace the hard drive and today I found the third crack to the case. I felt bad that my relatively new MacBook is cracked in three places because i I keep it on the coffee table or have it in a padded laptop bag and handled it carefully. I felt bad because I thought it was somehow my fault, that is until I searched flickr for macbook + crack and found many other computers with the exact same cracks.
There is even a flickr group called My MacBook Was Cracked By Itself
Turns out after looking around a bit that two of the cracks are cause by the magnets pressing the small ridges into the plastic and cracking the case.