I finally received the new chassis from Ace. I think what has been said is correct about the center chassis braces coming loose is what caused the crack in the first place.
I locked the screws down with red lock-tite, and have had 1 practice day, and 1 race day ( that I took 3rd in the A-main!) with nothing breaking, or screws coming loose.
I also have installed a aluminum diff mount, possibly the plastic one has enough flex to have contributed the problem. Its a little premature to tell for sure , but the original crack happened from normal racing, no terrible crashes, the difference being I was having problems with the screws holding the diff plate coming loose, and the plastic diff mount.
Here's some more pics of the cracked chassis and my new and improved S3:

http://homepage.mac.com/joefinn/S3/PhotoAlbum15.html