It is highly unlikely that al-Qa'ida will be able to obtain nuclear materials. You just can't carry it in your pocket. Not to mention that al-Qa'ida has no friends in any state that has access to nuclear materials.
Since the beginning of the American-Taliban war in 2001, the striking capability of al-Qa'ida took a nose dive. Before they were capable of attacking to seemingly impenetrable consulates in Africa, they were able to operate freely in America, and they were able to attack the USS Cole all among other things.
The next terrorist attack will not be a Muslim from Saudi Arabia, Pakistan, Syria, Jordan, Nigeria, Yemen, or even the United Kingdom or Jamaica, but they will be 100% American. Most experts agree with this notion.