Is / always the path of the system drive?
The basic answer is yes, although it depends a bit on exactly what you mean by "system drive". A few versions ago, Apple introduced what they call "Boot≠Root"; I can't find much documentation on it, but the basic idea is that OS X can load the booter file from one volume, then use a different volume as the operating system root.

For instance, when your startup volume is encrypted with FileVault 2, it actually loads the booter from a different volume (the hidden "Recovery HD" volume); the booter decrypts the startup volume, and then loads the operating system from the encrypted volume.

The rule is that / will always refer to the volume you are running the operating system from, just not necessarily the one that the boot process started from. This applies whether you're starting from an internal disk, FireWire disk, USB disk, NetBoot disk image, disk image,... whatever.