iTunes <-> iPhone. At sync time, a back-up is performed. Which data is included, which data is not? i.e. are songs (potentially redundant) backed-up so that a computer ends up having both the source file on the filesystem and the copy within the device back-up? Is anything on the iPhone filesystem not backed up? (i.e. on a Mac using Time Machine, some files are excluded from the back-up even if not all of them can be recreated upon restore - I lost my postfix config this way..)
Im pretty confident that songs are not backed up, as the last time I did a restore I had to reload all of the songs to my iPhone. It seemed to backup all config files for the phone, and messages, phone numbers, etc. Anything that can be downloaded and synced again from itunes was skipped.
I may be wrong here, but thats what I remember from experience.