> "Where's the line between two separate programs, and one program with two parts? This is a legal question, which ultimately judges will decide."
The types of links should be crystal clear from the licence, rather than relying on judges to determine fair use. Companies are likely to avoid the GPL due to the potential risk derived from this lack of clarity.
A terminal emulator and a child process will have different virtual address spaces, so you may choose that as the discriminating factor. A web browser and a web application will share address space. But a web browser and a web application are clearly two separate programs as well.
It isn't really a problem unique to the GPL. Any interpretation of a non-trivial license with conditions will have an element of "I know it when I see it". Software licenses especially due to the level of abstraction.