Could you point me in the direction of a court ruling establishing this general rule? I couldn't find anything after a quick search.
You didn't mention this part of the GPL FAQ (directly after the part you talked about):
> But if the semantics of the communication are intimate enough, exchanging complex internal data structures, that too could be a basis to consider the two parts as combined into a larger program.
Determining what constitutes "intimiate enough" sounds pretty difficult and open to interpretation: what's a "complex internal data structure"? If PostGIS is serialising its (presumably complex) representations of data and sending them over a socket to your program (and vice versa), then does that count?
I think it's also worth noting that "derivative work" is an established legal term; if it comes to a lawsuit, it's probably more likely that the judge will fall back on established precedent on what constitutes a derivative work and enforce the literal terms of the license as opposed to looking at FAQs written by someone who is not a party to the contract.