are you looking to move the bloated Apple Computer\MobileSync\Backup file? i had this same problem (my iTunes points to an external drive, but the default Backup Profile is still stored locally, and with a 64GB iPhone, it's killing my hard drive. here's what i found!
there are several posts about this, none are apple sanctioned it appears, all work arounds. read these:
the last one has the most simple directions i think. here's the direct link to his blog post (i can't take credit!)
basically you will "Create a symbolic directory link using the mklink command". no software to install, just a little dos editing.
best of luck!