It is confusing with different airlines rules. The problem is that at one airport, security is controlled by one airline, while at another, it is controlled by another airline.
For example, PIT is controlled, I believe by US Air. They may let you thru with that backpack, like DebbieB says, as a personal item, but then you fly back from ATL, which is controlled by Delta, and they won't let you use the backpack as a personal item. What then???
I do believe the FAA did mandate the carry on restrictions, and they take precedent over what any carrier states. However, the individual airline can enforce stricter guidelines if they so chose.