If you have a chance to get them on the net and have some German, there are one or two documentaries about Wacken. One is called Full Metal Village
http://www.flyingmoon.com/engl/news.html which is about the friendly relationship between villagers and metal fans, and the other one was on German TV and showed the amount of effort needed to run the festival (forgot the title).
FMV is really fun to watch, but there's hardly any concert footage in there, so don't say I didn't warn you.
