"As the copyright holder, ArsDigita has the right to release their software in whatever licence they want. The licences for their previous software was GPL, so it is GPL'd, they can not retroactively change the previous software. You can't be in violation, if you are releasing the software as the author as you do have the right to release the software as you wish."
There is an unfortunate implication of dual licensing in this fashion -- the difficulty of outsiders contributing to the project.
aD can re-release ACS 4.x under any license they wish, because they essentially wrote all the code for it. (so much for collaborative development..)
But what happens if a non-employee wants to contribute code to the ACS?
Because aD wants to "relicense" the code (or reissue it under a new, as yet undiscovered license), a developer who wishes to contribute _must_ turn over the copyright to aD. I'm guessing this requires submission of legal documents explicitly stating that the copyright gets turned over to aD to do with the code what they will.
In contrast, the GPL requires no such legal wrangling, because subsequent users of GPL code get the same rights as those that came before them.
So, the practical questions _I_ would ask are: If i'm a developer and I want to contribute to future versions of ACS, 1) how do I do it? Do I need to fill out a form waiving my rights to the material? and 2) can i guarantee that my code will only appear in the GPL'd version, not in a "commercial" version?
3) If the answer to #2 is "no", and my contributed code must go into both GPL'd and commercial versions, what sort of additional compensation or incentives do I receive, as a developer, by having my code included in a commercial project? (TINSTAAFL)
I suspect that these issues haven't yet been fully worked through. Regardless, it looks like a roadblock to effective collaboration.
