You can't change the iTunes behavior, as far as I can tell.
However, you can use a feature of NTFS to point that folder to another drive. This feature is typically called a "junction" or "symbolic link".
You can use this tool to create a junction:
http://technet.microsoft.com/en-us/sysinternals/bb896768.aspx
Download the "junction.exe" file from the link above and place on d:\
For example, let's say you want your iPhone backup to be D:\backup. You would do this:
1. close iTunes
2. Move the folder content c:\documents and settings\user\application data\apple computer\mobile sync\backup to D:\backup
3. Open a command prompt (CMD) and enter these commands:
(FOR WINDOWS XP)
D:\>junction "C:\Documents and Settings\username\Application Data\Apple Computer\Mobile Sync\Backup" D:\backup -s
(FOR WINDOWS VISTA / 7)
D:\>junction "C:\Users\username\AppData\Roaming\Apple Computer\MobileSync\Backup" "D:\backup" -s
NOTE 1: Change "username" to match your current username, of course.
NOTE 2: You can change the target D:\ for any other drive you have, like a external drive.
My Regards to author Robert Jacobson.
http://discussions.info.apple.com/message.jspa?messageID=10299450