Apple's soon-to-be-released Mac OS X 'Leopard' operating system has been awarded Unix 03 certification by The Open Group.

Apple joins only three other companies - Sun, IBM and HP - with certified Unix operating systems.

The Unix 03 certification entitles Apple to officially brand Leopard as a Unix operating system, but more importantly, the certification also guarantees software developers considering supporting the operating system that Mac OS X libraries, system calls, commands and utilities are all vetted as real Unix code.