Several users have reported problems when attempting to sync an iPhone, iPad or iPod with iTunes:
“The iPhone (or iPad / iPod) could not be synced because the sync session failed to start”
Try removing your iTunes back up files from your computer. Launch iTunes with your iPad/iPod/iPhone connected to your computer. You can delete your iTunes backup files by opening iTunes Preferences (Windows: Choose Edit > Preferences; Mac: Choose iTunes > Preferences), then click devices and click Delete all Backup. If you see duplicate backups listed, delete all but one backup. Then restart your idevice and your computer and reinitiate your sync.
Notes: Before trying this fix; make sure you back up your backup. The iTunes back up files on your computer are located at:
Mac: ~/Library/Application Support/MobileSync/Backup/
Windows XP: \Documents and Settings\(username)\Application Data\Apple Computer\MobileSync\Backup\
Windows Vista and Windows 7: \Users\(username)\AppData\Roaming\Apple Computer\MobileSync\Backup\
iTunes only updates backup files instead of creating a new file every time you sync or backup. Updating a file like this over time can cause it to become damaged / corrupted, and this may cause errors when you try to sync.