NOTE: I've turned on the comments for this one for feedback on this post.
Question: How would ISIS cross the "air gap" between their conquests in Iraq and Saudi Arabia?
Answer: It already has.
ISIS has become the leading supplier of the most potent drug in the world:
Further, Saudi Arabia is almost certainly one of their leading customers and they brought it on themselves.
More than half of Saudi Arabia's men are under 21 and most of those boys have been given a religious education in a strict literalist tradition. Further, they've been kept in time capsule, protected from many of the changes influences the rest of us.
To young men like this, ISIS is pure historical heroin. It's a jihad in the medieval tradition (made successful by using the techniques of open source insurgency).
This makes it far more compelling than the insurgencies against the Soviets in Afghanistan and the US in Iraq.
It also has all of the trappings of historical jihad. It has everything from the beheadings of unbelievers to war brides to the sprawling conquests over numerous foes.
To young Saudi men, it's an unstoppable and epic event.
A movement that can give a young man's life meaning. Meaning that can't be found in the hollow, timid commercial existence the west is forcing on us all.
For these young men, ISIS is a return to a virile Islam of the past.
An Islam uncorrupted by the grey, weak, and cowardly old men currently in charge. Men that deserve to die for their actions.
For them, ISIS is a chance to rewrite the rules of the world in a way they should be written. To be part of God's plan and to be part of destiny and not on the sidelines watching it happen.
Zealotry like this is intoxicating in ways that people living in our modern, connected world can't even imagine.
We're too jaded, medicated, and appologetic (of any strongly held beliefs) to understand real zealotry (particularly on a mass scale).
Of course, Zealotry like this isnt limited to Islamic youth.
Global history is replete with it although it has recently become relatively rare.
For example, if you want to get a feel for Zealotry in the Japnese tradition, give Yukio Mishima's fantastic book: Runaway Horses a read.