A/UX is Apple's version of Unix in the days when Mac OS X was not even a thought in anyone's mind.
It's a Unix implementation that is based on AT&T Unix System V.2.2 along with numerous extensions like streams and other features from V.3, V.4 and BSD 4.2/4.3. Of course it's fully POSIX compliant and gives you all the SYSV, BSD and POSIX compatiblity you'd expect.
The reason people think that A/UX might be the perfect operating system is the excellent grafting of MacOS 7 into the system, right in X. You can use it as a 'real' Unix, or let the system insulate you from it by using all the fabu GUI tools that Apple is famous for.
Being a product of bygone days, A/UX will only run the old M68000 CISC Macs, such as the Quadra's, Centris's and some Mac II's. The basic requirement is a 68030 CPU with a PMMU. The last machine Apple made that ran A/UX out-of-the-box was the Quadra 800. 8 meg of RAM is the minimun along with an 80 mb disk.
A few years ago, Apple sadly dropped A/UX completely in favour of AIX, and then went to MacOS X Server. Hopefully its legacy will live on. A/UX is not free (unlike MacOS 7.5 and older), and so while there are a multitude of machines able to run it, you cannot buy it and it's illegal to copy it.
Sources: the A/UX Faq: http://www.faqs.org/faqs/aux-faq/