But one gray area in particular is something like a driver that was
    originally written for another operating system (ie clearly not a derived
    work of Linux in origin). At exactly what point does it become a derived
    work of the kernel (and thus fall under the GPL)?
    
    THAT is a gray area, and _that_ is the area where I personally believe
    that some modules may be considered to not be derived works simply because
    they weren't designed for Linux and don't depend on any special Linux
    behaviour.
    
    Basically:
     - anything that was written with Linux in mind (whether it then _also_
       works on other operating systems or not) is clearly partially a derived
       work.
     - anything that has knowledge of and plays with fundamental internal
       Linux behaviour is clearly a derived work. If you need to muck around
       with core code, you're derived, no question about it.

Then you have things like (Open)ZFS and DTrace.