Carbon fibre is both light weight and bullet resistant. A friend of mine used to work for British Aerospace, he built the cockpits for Tornado fighter planes out of carbon fibre, and he told me that they could stop a 30mm (I think...) shell.
There are also glass/plastic laminates that can stop fairly large calibre rounds, GM just needs to start using some of these materials if they want to build safe, but lightweight, limosines.