I’ve just come across this nice piece by David Wheeler on open source licensing and what it means to be GPL-compatible. Anybody who needs to deal with this kind of stuff (myself certainly included) should take a good look at this article. Unfortunately, licensing is only just one piece of the whole puzzle: there is also copyright assignment side of things that one must deal with when managing or contributing to a multi-licensed open source project, adding more confusion to the already complex, legal aspect of open source.
One common scenario one may encounter:
Bob the contributor: hey, thanks for your work. I’ve made some neat change to your software. Here is my patch. It’s GPL, so feel free to commit it to your source.
John the maintainer: Thanks. But I can’t accept your patch without first having you sign this copyright assignment, which you must fax to this number, wait for a few weeks to have our laywer go over it, and …
Bob: Why!? I thought your software was GPL’ed. Why do I need to go though all that trouble just for one patch!? Just take it.
John: Yes, but…(John’s software is dual-licensed with GPL and a commercial license for his own company.)
Things get more complicated when Bob’s patch was derived from another GPL’ed software that someone else wrote… Go figure.