How can I know which GUID represents which app in my iTunes backup of my iPhone?
This used to be possible with Erica Sadun's mdhelper program.  Unfortunately, it looks like something has changed in the way Apple stores backup files w/ iOS 4.2, and the program cannot achieve the same results (since it finds no mdbackup or mdinfo files).

On the bright side, a new tool named iPhone / iPod Touch Backup Extractor has emerged.  It correctly translates the GUIDs to names in my iOS 4.2 backups, and allows the files for each app to be extracted together.