We can't underestimate the impact that 9/11 has had on NORAD. Whereas NORAD was focused on the assumption that it would be a foreign-based threat coming from outside of North America, 9/11 proved that it could be a threat coming from within North America, and it wasn't a defence threat: it was a plane, which isn't a threat normally and which became a weapon.
When you go to Colorado Springs, what you're hit with is the number of reminders—pictorial reminders, statues—commemorating the events of 9/11 and the association with USNORTHCOM and their motto, “We have the watch”. There is this real concern that something—call it Rumsfeld's “unknown unknowns”—is going to come out of nowhere and attack the homeland.
Now that the focus is squarely on the homeland, NORAD is making sure that they consider all possible scenarios for what could be a threat to the defence of North America.