Actually this is a perfect technique for Al Qaeda: By not attacking the Royals directly they insure that their funding continues; by confining the attacks to foreigners within the Kingdom they will ultimately cause the House of Saud's downfall -- by Westerners!
It's a perfect leveraging of their rather limited power.
I question this. The "Westerners" (a.k.a. the rest of the world) may be a contributing factor to this but the finial push, as it were, will come from w/i. The Saudi's hold will become ever weaker and unsteady, a native politican (civilian or cleric) will stir up the people demanding "Do we want leaders who (fill in the blank)...Then it's time we rise up and (fill in the blank)..."
And farewell,House of Saud.
...So that the westerners will occupy Saudi Arabia and anger the entire Muslim world into revolt? On a scale of 1 to 10, I'd give it a 6. Depends on how sane you think Al Qaeda's leaders are.