I am not a lawyer.
There was talk on the WP Hackers list recently that discussed how WordPress plugins had to be released under the GPL, citing this FAQ:
If a program released under the GPL uses plug-ins, what are the requirements for the licenses of a plug-in.
It depends on how the program invokes its plug-ins. If the program uses fork and exec to invoke plug-ins, then the plug-ins are separate programs, so the license for the main program makes no requirements for them.
If the program dynamically links plug-ins, and they make function calls to each other and share data structures, we believe they form a single program, so plug-ins must be treated as extensions to the main program. This means they must be released under the GPL or a GPL-compatible free software license, and that the terms of the GPL must be followed when those plug-ins are distributed.
If the program dynamically links plug-ins, but the communication between them is limited to invoking the `main’ function of the plug-in with some options and waiting for it to return, that is a borderline case.
That seems to be at odds with FAQ #10 on the Mambo web site:
10. I have written a Component, Module, Template for Mambo. Do I have to release it under the GPL?
No The GPL allows you to write your own extensions for Mambo and to release those extensions under whatever license you chose.
Has anyone delved into this before? Please educate me.