It's a distant possibility, I know, but check this out:

Saudi Arabia (majorly Sunni) sends troops to Bahrain to help the Sunni leadership put down a rebellion that largely involves the Shiite underclass.

Now the Bahrain (and Saudi) military is breaking from the Tunisian and Egyptian mode—instead of capitulating and negotiating, they're showering fire and live ammunition on protesters. And not just any old protesters, but mourners at a political funeral, which gives them a special kind of cultural/moral protection. From the NYT:

MANAMA, Bahrain — Government forces opened fire on hundreds of mourners marching toward Pearl Square Friday, sending people running away in panic amid the boom of concussion grenades. But even as the people fled, at least one helicopter sprayed fire on them and a witness reported seeing mourners crumpling to the ground.

It was not immediately clear what type of ammunition the forces were firing, but some witnesses reported live fire from automatic weapons and the crowd was screaming “live fire, live fire.” At a nearby hospital, witnesses reported seeing people with very serious injuries and gaping wounds, at least some of them caused by rubber bullets that appeared to have been fired at close range.

Furthermore, the fire comes from the army, not the police (harder Saudi connection) and the Saudi-backed government has made life difficult for doctors and would-be blood donors "because the government had issued orders against helping protesters."

Maybe the Bahraini gambit will work—maybe their brutal tactics will crush the dissent that turned governments upside-down in Egypt and Tunisia. But... what if it doesn't work? What if the Bahraini government, backed by the Saudi military and Saudi government, gets its ass kicked? What will that mean for the people of Saudi Arabia, where the government has seemed immobile and invulnerable for generations?

Meanwhile, at least one Saudi prince has called for reform. (Admittedly, he's a family outsider and career reformist—but somebody's got to be the first one in the pool.)

This matters because many, many observers (including Slog commenters) have insisted that no matter what happens in the Middle East, the House of Saud is not going anywhere. But the House of Saud has a major stake in the Bahrain situation. And if that goes poorly for the government, it could open a fissure in the House of Saud's perceived invincibility.