But the threat has wider implications. Al-Qaida has also warned it will attack Saudi interests in Pakistan, where Saudi Arabia's intelligence services are leading efforts to bribe and cajole the Taliban away from its alliance with Osama bin Laden.
The new threat to Saudi Arabia was made by Nasser al-Wahayshi, leader of the Yemen-based al-Qaida in the Arabian Peninsula, or AQAP, in a video posted online and translated by the U.S.-based monitoring group SITE.
"If you can flee with your skin, then do so," he told the Saudi royal family. "By Allah, they (al-Qaida fighters) will climb your walls and will come to you where you do not expect. …Our heroes have woven their grave-clothes with your blood."
The threat was given weight by an Aug. 27 attempt to assassinate Prince Mohammed bin Nayef, Saudi Arabia's deputy interior minister and the commander of counter-terrorism forces.
A Yemeni jihadist, Abdullah bin Hassan bin Taleh Asiri, was able to get close to the prince by pretending he was about to surrender to him. Asiri had explosives hidden in his body and intended to detonate them when he was greeted by Prince Mohammed.
But he stumbled several feet from his target and the bomb exploded prematurely. The prince received minor injuries.
The bombing was the first direct attack on Saudi Arabia's royal family and the first assault on the Riyadh government since the Interior Ministry in Riyadh was car-bombed in 2004.
Had the assassination bid succeeded, it would have been a major coup for al-Qaida at a time when its campaign in Saudi Arabia has been crushed, largely by Prince Mohammed's forces. More than 150 foreigners and Saudis perished in the 2003-05 violence.
Since then, al-Qaida has regrouped in Yemen, where the beleaguered government of President Ali Abdullah Saleh is pinned down fighting a tribal rebellion in the north and separatist unrest in the south.
The Americans and Saudis fear al-Qaida will exploit this crisis to rebuild and unleash new attacks on Saudi Arabia.
The Washington-based Jamestown Foundation, a think-tank that monitors global terrorism, reported in a Sept. 17 analysis that the attempt to kill Prince Mohammed "demonstrates the continuity of al-Qaida in Saudi Arabia despite its decline over the past few years."
It added that "resorting to such a tactic also demonstrates the inability of al-Qaida to implement attacks that require major logistical support, such as targeting residential compounds or oil facilities."
But it noted: "By using bases in Yemen, al-Qaida might be able to mount high-profile, low-cost assassinations to destabilize the regime and demonstrate that it is still a strong organization in Saudi Arabia, an essential part of invigorating its recruitment efforts."
Osama bin Laden and his deputy and eminence grise, Ayman al-Zawahiri, have escalated their verbal threats against the Saudi royals over the last 18 months.
This presumably was because of Riyadh's success against the jihadists, but King Abdallah's call for dialogue among Muslims, Christians and Jews seems to have particularly stung al-Qaida's leadership.
Al-Qaida ideologue Abu Yaha al-Libi exhorted Muslims to kill the monarch, branding him "a reckless tyrant" and "lunatic apostate" in videotapes issued in May and July 2008.
But there is a more profound reason for this new fixation with toppling the Saudi monarchy: Riyadh is currently engaged in a global effort to cripple al-Qaida by sabotaging its funding and recruitment.
This effort, spearheaded by Saudi intelligence agencies dishing out large amounts of money to influential Muslim leaders, is currently centered in Pakistan, a longtime Saudi ally and currently al-Qaida's main sanctuary and operations center.
According to the Texas-based private intelligence consultancy Strategic Forecasting, or Stratfor, King Abdallah is reported to be "personally involved behind the scenes in efforts to pressure the Taliban to break free from al-Qaida."
If that was achieved, al-Qaida's leadership would have no sanctuary. To this end, Stratfor said, Riyadh has sponsored trips to Saudi Arabia for Taliban luminaries, supposedly making pilgrimages to Mecca, "to twist arms and offer cash."