I was really surprised when one of my apps was rejected by Apple when all I did was update the icon and the app have past previous reviews with flying colors. It seems like there's a new rule about data you store in the /Documents folder.
All files in the Documents folder are backed up to iCloud and such files that can be re-downloaded or re-created should not be stored there. For obvious reasons, to not waste those precious 5 GB we have by default. However, if you put your data in Caches folder or tmp folder, it might be deleted in low storage situations so there is a new solution from iOS 5.0.1.
This allows you to put files that you generate, like a read-only database, to the Documents folder without having it backed up to iCloud.
Here's a snippet from iOS Developer Library
How to set the attribute
- (BOOL)addSkipBackupAttributeToItemAtURL:(NSURL *)URL
const char* filePath = [[URL path] fileSystemRepresentation];
const char* attrName = "com.apple.MobileBackup";
u_int8_t attrValue = 1;
int result = setxattr(filePath, attrName, &attrValue, sizeof(attrValue), 0, 0);
return result == 0;