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.
This is a major change (as I see it) in licencing of ACS and understandably some research and thinking should be done on the communitiy's part.
Of course, we could always forward it to RMS (who I'm guessing that didn't talk to before releasing this) and see what he thinks about it :)
