On your Mac or PC, if you choose to store backups locally on your computer, they are located in very hard to find places.

On the Mac, they are located here: ~/Library/Application Support/MobileSync/Backup Which you can easily navigate to if on Finder, you press CMD-Shift-G (Go to folder).

On the PC, the backups are located here: C:\Documents and Settings\YOUR_USER_NAME\Application Data\Apple Computer\SyncServices\Local

Hope this helps!