Mac OS X 10.5 Leopard has achieved UNIX 03 certification. The certification is granted by the Open Group, an international group which ensures conformance to recognised standards.
This achievement puts Apple into an elite of certified companies - the other three which have Unix 03 certification are Sun, IBM, and HP. Apple even has a certificate to prove its achievement, which you can see here.
The UNIX 03 specification covers libraries, terminal interfaces, commands, the C language and more - essentially all the components that make up currently not on Mac UNIX applications.
Apple's achievement of qualifying for recognition as real UNIX may kick-start the company's move into the enterprise, because UNIX developers will be happier to build applications for Leopard now.