Cracking laptop Gromit
Apples MacBook is generally a tough customer. Designed to be used by students is can take the odd knock or two and keep on keeping on. So why, when some users treat their machines like they were swan coated crystal goblets filled with unicorn tears do they persist in developing mysterious cracks in the casing. And why do these cracks always seem to occur in the same places, in both black and white MacBooks?
Can you spell "design fault"
The case cracking is in fact caused by the closing of the laptops lid where the two small raised ridges on the lid meet the top case, eventually causing cracks to appear due to the top case casting being made too thin at these points.

Later MacBooks have proved more resistant to the cracking disease although they are not entirely immune. New style unibody MacBooks are entirely unaffected.
So, given that these unsightly cracks are not your fault, what can be done about them. Chances are that your MacBook is well out of warranty by now. But wait, what's this… Apple have recognised the issue (in part by actually redesigning the case part) and will man up to the issue and replace any affected top cases free of charge. However old the MacBook.
Sweet!
You can take affected MacBooks to Apple resellers or to Apple retail stores. In the case of the latter your MacBook may well be repaired on the spot or at most in a few days. Resellers will have to order parts into stock and this may take a week or so.
But still. New MacBook top eh?