Mac OSX is, as of version 10.5 (Leopard), no longer an Unix-based OS, but a true Unix version. On May 18, 2007, Mac OS X 10.5 received the UNIX 03 certification, meaning that Leopard conforms to the Single UNIX Specification Version 3 (SUS), which defines how components like the command shell, the compiler, and C APIs should function and behave.
This means that in the future, enterprise customers, who need ‘real’ unices to run their applications on, can also add Mac OS X 10.5 machines to their list with possible choices, as applications written for one Unix 3 platform are easily portable to others. This also means that developers can now easily start using Mac OS X 10.5 machines (like the iMac) to do their development.
The only other players in the Unix field that have Unix 3 certification are IBM (AIX), Sun (Solaris), and HP (HP-UX).
The certification, at this point, is only for the Intel-based version of Mac OS X 10.5
Welcome to the big world of major Unix players Apple!