If they can smuggle PETN onto a plane, imagine the damage they could do to simultaneous attacks on the subway and commuter trains?
I have been thinking along the same lines, this attempt does not have the hallmarks of an AQ stile attack. I am sure they would have been happy if they brought the plan down but in terms of totally beating security the mission was a complete success.
The next step would seem to be to lunch multiple coordinated strikes around the world. However this may also just be a distraction to keep us focused on the airline industry when the real target is something completely different.