Re: I think Apple owns Unix now anyway @lars
You are so wrong in your suggestion that there is no AT&T code in AIX. Also, you are wrong about people wanting to pay for UNIX branding. Look at the Open Group website, and see which UNIX variants have been put through the various UNIX test suites (which costs quite a lot of money). IBM, Sun (as was), HP and Apple have all paid the money, and achieved the certification.
IBM has a SVR2 source license and AIX was very clearly derived from AT&T SVR2 code. It was not written from the ground up. I've worked in IBM and had access to the source code, and I have seen parts of the code that are clearly related to AT&T UNIX, complete with the required AT&T copyright notices. This was a long time ago (early '90s), but they were there.
For Power systems the current AIX can be traced back to AIX 3.1, released on the RISC System/6000 in 1990. AIX 3.1 itself was derived from the code that IBM had for the 6150 RT PC, and this was a direct port of SVR2, mainly by IBM but aided by the INTERACTIVE System Corporation, who had also worked on PC/IX for IBM. Reports of the Kernel (in places like Wikipedia) being written in PL/I or PL/8 refer to the VRM, not to the AIX kernel.
I admit that there has been a huge amount of code added in AIX over the years, but it is still a genetic UNIX. How much code is related? Maybe you should ask SCO. They've seen the AIX source.
The same is true for SunOS/Solaris. I was working for AT&T when SVR4 was released, and I can say with absolute certainty that Sun0S 4.0.1 was the same source code base (again, I had access to the source code) as AT&T's SVR4.
Sun were one of the principal members of UNIX International and the Unified UNIX programme that attempted to standardize UNIX in the late 1980's with AT&T, ICL, Amdahl and various other vendors long gone. I still have the notes from the developer conference. Prior to this release, SunOS 3 and earlier was based on BSD 4.2, with enhancements added from 4.3.
I am not so clear about HP-UX, but I know that HP had a direct UNIX V7 port on a system I'm sure was an HP 500 in the early 1980s, although I can't find any references (it was pre-Internet). Wikipedia says HP-UX was derived from System III. HP (and in fact IBM and DEC) were in the Open Software Foundation that was set up in opposition to the Unified UNIX. They had their own UNIX called OSF/1, which had a common code base that was taken from DEC and IBM versions of UNIX. The tension between UI and OSF was known as "The UNIX wars".
Time moves on, and of course there is no feedback from the vendors back into the main tree, so of course the different versions diverge, but I am sure it is safe to say that all three of these are genetic UNIXes, and they all have achieved UNIX branding at various levels. They can all be called UNIX as per the branding rules, but in this day an age, this is not really important. UNIX as a unified OS (much to my regret) is largely a has-been.
My biggest fear is that without some form of standardization (like the Linux Standards Base which is mostly ignored) Linux will go the same way.