id's John Carmack made a post over at Slashdot clarifying some issues with the GPL license under which the Quake 1 source code has been released (story) and perhaps raising a few as well:

We are the copyright holder of all works, and we can release any part of it under any license we choose.

Completely aside from that, I think it is still unclear exactly where the GPL wants the separation of code and content. Few would argue that every document read by a GPL word processor would be covered by the GPL, and most would place maps entered by Quake into that catagory, but things can quickly get murky.

Quake game mods are written in QC, but turned into data to be processed by the main code. I think the spirit of the GPL would want that code to be released, but it is only a small step from there to saying that every program loaded by a GPL operating system must be GPL, which is clearly not the case.