Al-Qaeda attacks have always been spaced out 18-24 months apart, so I wouldn't get too excited about the fact that there hasn't been another one just yet.
Not for lack of trying. One instance we all know about is the shoe bomber. Al Qaeda, no? Prevention provided by alert flight attendants and passengers.
I tend to think there have been other attempts that have been foiled that we don't know about.
I tend to think there haven't.