I'm not fully convinced that Al Qaeda is capable of mounting a major attack in the U.S. Their organization has been hammered by the war in Afghanistan, and the arrest of a number of senior operatives over the past year. Al Qaeda, as an organization, is still in the rebuilding mode.
And that raises an important point that most commentators are missing. With only one major success since 9/11 (the Bali bombing), Al Qaeda had to prove that it's still a viable organization. To achieve that, they're going after the "low hanging" fruit in the terrorist orchard, hitting soft targets in countries where security is lax to non-existent.
The good news here is that Al Qaeda doesn't appear ready to stage another "spectacular"--at least not yet. Those strikes require extensive planning and preparation. With many of their senior guys dead or in jail, that's become a more difficult proposition. Al Qaeda also seems concerned with the long lead time needed to plan those ops. The more time spent planning, the great the chances of being detected by U.S. intelligence.
the bad news? Security is too lax in too many countries, so there are literally hundreds of potential targets. And Al Qaeda can make a living off those attacks, rebuilding its ranks and preparing for the next big one.