If this was a terrorist attempt to down a plane and it well could be, I am more puzzled than ever about their mental state. What exactly do they think they are going to accomplish with the downing of one plane? The downing of 10 or 20 planes would scare and cripple us for a while, a nuclear blast in the heart of any city would certainly give us pause, but one plane? Are they feeling irrelevant and they just want to remind us that they are still out there?
You are more correct than you think! Terrorism is not about MAJOR actions. It is about the slow building of fear and disillusionment with the ability of the government to protect you. The ultimate goal is to "convince" you to make the government change thier policies (e.g. surrender to terrorist demands).
Thus even "failed" attempts (when publicized) can greatly benifit the terrorists as the fact the "they are still there, plotting" and "it's only a matter of time" both aid the terrorist - they want us to live in fear and to dread the next newscast!
So, yeah, targeting "one plane" - thus proving that they CAN is entirely possible.