Thanks for linking to my post, Shamus.
I can tell you right now, staying in the castle was the LEAST creepy and the most exciting thing I've probably ever done in my life!
As far as how you get to stay there, according to the CM's when we were there, they said it is only available as a prize. Like Travelocity might "buy" it to give away in a contest. (I'm not sure about that "buying" part, that was just the example they gave.) Or other departments of Disney might use it to give away as a prize.
They told us the reason Tom & Suri got to stay there is b/c Tom signed a 3-movie contract w/one of Disney's companies and put it in his contract that Suri could have her bday party at Disney AND they'd get to stay in the castle. So, I guess it can kinda sorta be bought if you're making millions of dollars for Disney. (Or maybe it can be given as a gift--I heard Michael Buble stayed there recently.)
Our VIP CM told us that if a dignitary or Oprah or a random bazillionaire asked to stay in the castle, it would be impossible. It's not for sale.
So, it's truly a once-in-a-lifetime event! I literally get giddy every time I look at that post or think about that night!