Actually there are composite plastics that are bulletproof. Kevlar is is a good example of one of these composites. The problem with metal is that it has a lot of give relative to its strength so when something hits it with a lot of speed and heat like a bullet the metal will bend at the point and will weaken until it breaks through. Composites work differently. And the iBook was made with such materials in its case to offer better protection for components, not to say that it is bulletproof by any stretch but it does have practical applications.
Well, your sort of right. The composite material you are thinking of is boron carbide, which is used as protective inserts in Kevlar vests. The composite is used in a plate based form, very similar to how the plastic in an iPod is formed (or whatever generic product that uses plastic). Kevlar is a synthetic fiber derived from plastic that is woven into a vest, very much like fabric. These vests have slots where SAPIs (protective inserts) can be inserted into it, usually in the front and back. Kevlar is fairly different from such composites.
When you think about it, it's very similar to the old ways of using a chain mail and breast plate combination. Kevlar is like the chain mail, and the SAPIs are the plate. Full plate was too cumbersome and expensive for most soldiers, so they only used a breast plate which protected all the vital organs in the torso. Very similar to the current situation; while you could use full composite plate in body armor, it would be difficult to move, and too expensive to issue to thousands of soldiers.
Kevlar fabric is used to primarily protect from heat/fire (firefighters use it), and projectiles such as shrapnel and small caliber bullets. It does reduce the lethality of all bullets but there are no guaranty that it would protect much further above 9mm (depending on thickness). It also has a problem of transferring a lot of kinetic energy which can, if your unlucky can hit a vital organ (can even stop your heart if hit hard enough), which is the problem from heavy slugs. This is why they use plate inserts which can handle rifle rounds pretty effectively. Also explains why most soldiers that are killed by gunfire are hit somewhere outside of the plate's protection.
On a side note there really no such thing as bulletproof. That's why they now call most "bulletproof" glass "bullet resistant", and call the vest a "ballistic vest" instead of bulletproof vest. It would be very difficult to create armor with a 0% chance of failure.
