Dave
This is my take on how it works. As flybyjim stated, for general camping you can prepay but it is first come, first served. Camp Scholler opens on Jul 2. If you or someone you know who lives in the area can get to the campground on 7/2 or the date you selected, you can stake out your space but you start paying ($28/day) from that day whether you're there or not. In my case, I prepaid from 7/21 til the end of the show. If I don't get there until the 23rd the 2 days I missed are not refunded ( at least it wasn't a couple years ago when I arrived late). If you have deep pockets, you can select (online only) a reserved electric/water site starting 7/2. Even at $70/day, these go relatively fast. As I said, this is my understanding on how it works. You can always call EAA and get definitive answers. Camp Scholler also has the Express Arrival program, so if you prepay you will receive credentials in the mail allowing you to bypass registration lines. Also, if you're unaware, if you leave early, you can get a refund for unused days (3 day minimum charge).
Rick