Leopard is UNIX compliant

Something a lot of people seemed to have missed from yesterdays WWDC keynote, Leopard will be the first OS X to be fully UNIX compliant:

Leopard is now an Open Brand UNIX 03 Registered Product, conforming to the SUSv3 and POSIX 1003.1 specifications for the C API, Shell Utilities, and Threads. Since Leopard can compile and run all your existing UNIX 03-compliant code, you can deploy it in environments that demand full conformance — complete with hooks to maintain compatibility with existing software.

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