Where is the dividing line (mount point) of the two iOS partitions?
/ is the mount point for /dev/disk0s1s1; /private/var is the mount point for /dev/disk0s1s2.



disk0s1s1 is reserved for iOS system files. It is equivalent to OS X's /System and /Library directories. It is the smaller of the two partitions (the size can be set using Redsn0w or Sn0wbreeze during the jailbreak process). The default size is set to 1 GB for iOS 5 and around 600-700 MB in previous versions.

disk0s1s2 is reserved (as of iOS 1.1.3) for user data (3rd party applications, music, photos, etc.). It is the larger of the two and varies depending on the flash memory available (which is dependent on the device purchased).

The layout of the disk is stored in /etc/fstab, which is also used by jailbreak tools to make the system partition writable.


Source: iOS Forensic Analysis
Additional reading: The iPhone Wiki