A Developer's blog

How to change the iPhone backup location (Windows)

1. Close iTunes
2. Move the backup folder (residing at C:\Documents and Settings\<YOUR_USERNAME>\Application Data\Apple Computer\MobileSync\Backup” to a place where you wish to sync your backup to (e.g. D:\iTunes_backup\Backup)
3. Download Junction.zip (get it free from http://technet.microsoft.com/en-us/sysinternals/bb896768.aspx) and extract junction.exe
4. Open Command Prompt and type the following commands (assume you extract it to the desktop folder):
   cd Desktop
   junction “c:\Documents and Settings\<YOUR_USERNAME>\Application Data\MobileSync\Backup” D:\iTunes_backup\Backup
   (ensure that there’s no Backup folder in c:\Documents and Settings\<YOUR_USERNAME>\Application Data\MobileSync when you are typing the above command if not it will say that the path exists)

Basically the junction type creates something like a symbolic link (like in *nix operating systems)