Same thing happened to me 3 days ago, I was playing Ridge Racer on the world explorer, and suddenly the screen froze with the words "loading" on it, so I open the tray and I found a big crack on the middle of the disk.
It was not the first time that my console does this, it happened before with my copy of Gears of War, but at that time I thought that it was my fault, because the xbox was on vertical and I accidentally moved it with the disc inside, so from that day I decided to play with the xbox in a horizontal position, as I did with this RR6 game, so I don't know how it cracks the cd this time..
Maybe playing too much with the same disc might be the trouble.. I was playing more than 8 hours on a day.
Anyway It sucks, but fortunately my Gears Of War, Halo 3, Lost Planet and other more expensive games, seems to look fine, so I'm buying this game again for $20 bucks, maybe this time, it`s going to last until I finish the damn thing..