I was only slightly connected with the development of the POSIX standard, and as has been mentioned here, AT&T was a full participant.

The real conflicts were in a related area and at the time were termed The GUI Wars. Basically, it was AT&T and Sun pushing OpenLook and everyone else pushing Motif.  I seem to recall that the Motif folks (Open Software Foundation) more or less forced IEEE to just accept Motif as a standard.

I was on a different committee that was trying to standardize a universal GUI interface that could work on any GUI, including Mac, Windows, Motif, and OpenLook. My product, XVT, was the base document. We never got past the draft stage.

Marc