ITunes Backup Directory
iTunes stores a backup of your iPhone (which it makes sometimes when you sync) in the following locations. According to Apple's Documentation
Mac: ~/Library/Application Support/MobileSync/Backup/ Windows XP: \Documents and Settings\(username)\Application Data\Apple Computer\MobileSync\Backup\ Windows Vista: \Users\(username)\AppData\Roaming\Apple Computer\MobileSync\Backup\
Remember, it only keeps the most recent backup. So you should backup this directory with a different solution (e.g. Time Machine on Mac), else it may not be useful in the event of a data loss issue.