Apple hasn't publicly acknowledged that there's a problem with Macbook cases cracking, but insiders say the company sent internal memos about how to address the problem.

A number of complaints have been lodged about cracks and stress fractures appearing in the Macbooks' plastic cases. There's even a group on Flickr where hundreds of users have posted pictures of their cracked products.
Apple hasn't made any public comments about the issue – which could develop into a class action lawsuit – but apparently it has acknowledged it inside the company.
Apple Insider reports that the organisation issued a bulletin to authorised service providers back in March, telling them to replace the bottom casing of devices that come in cracked – a reversal of an earlier directive. Previously, Apple refused to cover this issue.
The web site reports that the new directive identifies the four areas most likely to crack as the area below the palmerst and trackpad, areas around the I/O ports, the back rear corners and the back rear ventilation panels.