Was just reading Judge Debevoise's rejection of USL injunction on Berkeley. Sounds to me like in the particular case of Unix, assuming APIs were somehow made copyrightable in general from precedent after some cases like Oracle v. Google, BSD would probably be okay. He was suggesting USL wouldn't have luck because they didn't put copyright notices on lots of code and that mattered back in the pre-Berne convention days:
http://sco.tuxrocks.com/Docs/USL/Doc-92.html