Obviously the debate here hinges upon how we define "peril." I'd define "peril" to mean a threat that could destroy or seriously alter the character of our country. By that definition, I don't believe that al Qaeda poses any peril to the United States right now (an important caveat I'll flesh out below). Currently, al Qaeda's threat is limited to sending out one poorly-trained airline bomber every few years. This doesn't come close to the threat that, say, Palestinian suicide bombers posed to Israel in 2001-2002, when ordinary people feared for their lives every time they stepped on a bus or went to a restaurant.
You can't measure a terrorist threat solely by the number of lives it claims. Terrorist murders are not like highway fatalities -- they terrorize people. (Which is why the attack on the CIA in Afghanistan noted by Marty, while far more strategically consequential than an airplane attack, created a tiny proportion of the media hysteria.)
On the other hand, a society does have some control over what degree it allows itself to be terrorized. And Peter's point remains, in my view, compelling -- al Qaeda's ability to carry out mass casualty attacks upon U.S. citizens appears to be radically diminished, in part due to the success of the American war in Afghanistan. The prospect of al Qaeda carrying out a Abdulmutallab-type attack every few years -- a crude attack by a poorly-trained half-wit -- cannot imperil America unless we choose to paralyze ourselves with terror.
Now, there is one way in which I would agree with Marty that al Qaeda does pose a great peril to the United States: the prospect of acquiring a biological or nuclear weapon. That day appears to be far off, but the consequences, should it ever happen, would dwarf even 9/11, let alone the rather pathetic attacks that have followed. The campaign to destroy al Qaeda has to be considered principally with that reality in mind.
In the meantime, I agree with Peter that we should celebrate our success against al Qaeda since 9/11. For Americans to live in fear of this shriveled little band of murderers would be to hand them an unearned victory.