The U.S. Transportation Security Administration (TSA) lets you keep your notebook in its carrying case if it meets its Checkpoint Friendly design criteria. You can read about this policy in the TSA blog...

"Checkpoint Friendly" Laptop Bag Procedures (08.15.2008)

An Update on Checkpoint Friendly Laptop Bags (09.08.2008)

...which provides simple illustrations of the three basic styles: Butterfly, Trifold, Sleeve.

This policy started in August 2008 and I've been looking around for a TSA Checkpoint Friendly bag or backpack since then. As with any bag, there's a lot to consider when buying a notebook bag to make sure it fits your needs as well as the TSA's. I took a quick look at one recently and found that while the basic design met the TSA requirements, the bag (which will remain nameless) had no protection at the bottom of the backback area where the notebook is stored. This means that the side of the notebook is vulnerable to impact everytime this TSA compliant backpack is placed on the ground. If you look at well designed notebook bags and backpacks, the bottom of the notebook carrying case area is away from the actual bottom of the bag itself with either air or some shock absorbant material in between.

The lesson from this bag is to really make sure the overall design meets your requirements. Don't just check the TSA compliant aspect of it.