In iOS 4.0 Apple has redesigned the backup process.
iTunes used to store a list of filenames associated with backup files in the Manifest.plist file, but in iOS 4.
You can find information and a little description of the MBDB/MBDX format here:
http://code.google.com/p/iphonebackupbrowser/
This is my application to browse the backup files. I have tried to document the format of the new files that come with iTunes 9.2.