in Mac OS X your iPhone files are backed up at the following location:
The (~) symbol signifies your home directory, which is the same place all your other personal documents are stored.
Phone Backup Location for Windows 7, XP, and Vista
Windows XP stores all of your iPhone backup files in this location:
C:\Documents and Settings\user\Application Data\Apple Computer\MobileSync\Backup
Windows Vista and Windows 7 backs up the iPhone files to here:
Obviously if your main drive is not C: then you’ll have to change that, same goes for ‘user’