Sometime before the end of the year (October being the current guess), Six Apart will be releasing a GPL version of MT, termed “MTOS” (Movable Type Open Source). WordPress is also licensed under the GPL. So, once Movable Type is available under the same license as WordPress, what are the ramifications?
The ramifications are interesting, because while both will be available under the same license, there will be one key difference between MTOS and WordPress. And that is that MTOS will have one contributor. Six Apart will be accepting patches from third parties, but only if they sign over copyright of such contributions to Six Apart. Contributions to WordPress are owned by the person who contributes them.
The reason Six Apart requires this, is so they are able to distribute a non-GPL version of Movable Type. Otherwise, the viral nature of the GPL would require that the “pro” version of Movable Type also be GPL licensed.
The interesting ramification of this is that while code can flow from MTOS to WordPress, Six Apart can’t allow WordPress code (or code from any other GPL’d project) to end up in MTOS, unless it gets the author (or authors) of that code to sign over copyright of the code to Six Apart.
Note that this also allows Six Apart at any time in the future to say “As of today, we are no longer releasing a GPL version of Movable Type.” And that would require that someone fork the code in order to proceed with development. WordPress can’t easily do that, as it is not owned by a single legal entity.