Plastic isn't good enough for the new thinness.
Aside from the Apple MacBook Pros and select premium PC notebooks such as the Voodoo Envy and Dell Adamo, nearly all computers today are made from plastic casing.
While plastic is a good material for notebooks because it is light and easily molded, it's not particularly rigid. This is a problem that some computer manufacturers are now finding, according to comments made by Broadpoint AmTech analyst Doug Freedman.
CNet quotes Freedman, saying, "Early production units being built in plastic, with the bottom case being plastic, are cracking. … So, to get that really thin form factor that they're after, they're probably going to have to go with a metal case."
Computer makers are currently trying to make thin and light laptops based on Intel's CULV chips (consumer ultra low voltage), which pave the way for affordable thin and light notebooks. The problems aren't to do with the chips, however, but rather the form factor that the new hardware facilitates.
With apparent problems with using plastic, switching to metal would likely drive up prices of the notebooks that were meant to come into a price point that's affordable for the mainstream consumer.
It'll be interesting to see the type of design solutions that ODMs and OEMs come up with to try to solve this problem.