Mac OS X
The original location of the Backups is in a folder called MobileSync
This folder is located in: ~/Library/Application Support
And to do move this folder's location, you will have to:
Copy the folder to the new location (for example at the root of an external hard disk drive (HDD)
Go to ~/Library/Application Support/MobileSync/
Remove the folder "Backup" within or rename it "BackupOld" if you want to keep it
Open Terminal
Type the following command:
ln -s /Volumes/<Your External HDD>/MobileSync/Backup ~/Library/Application\ Support/MobileSync/Backup
Close Terminal
Launch a backup, see if it has been properly located and iTunes sees is as the latest
Voilà, saving precious disk space on Mac OS is easy!