The US Homeland Security department's 15 attack scenarios (Download 15-attacks.pdf) are now available. All of the attacks listed are high casualty events (biological, chemical, radiological). As we have seen, these large attacks are extremely rare.
The problems with this list are numerous. First, events of this type should be categorized as black swans -- unpredictable events. As the security "big thinker" Bruce Schneier points out in his book "Beyond Fear", defense against black swan scenarios are a waste of limited resources. The only thing of potential value is the development of a limited generic response and analytical capability. Second, al Qaeda's objectives for a big attack against the US were met by 9/11. The US is involved in a regional guerrilla war that has polarized the Islamic world (this was confirmed by the recent al Qaeda strategy document, "Idarat al-Tawahhush"). Finally, al Qaeda's capability to launch a well planned high casualty event in the US has been significantly degraded (the cohesive group size necessary to launch an operation of this type is larger than the survivable level we see today).
My recommendation: We need to understand what is working today in Iraq. As my analysis indicates, it is small attacks against infrastructure and markets that cause substantial economic and social dislocation. This is a strategy that has been recently ratified by bin Laden.