Apple is proprietary. Linux is open-source. Its an operating system free from virus, worms, spyware and malware. And if a program stops working, it doesn't take down a server with it. Companies still prefer to Windows NT. Reduced maintenance costs and up-time reliability are a consideration.
A lot of Mac OS X is open source, especially at the Unix level. For interoperability in proprietary products, Apple makes extensive use of open standards for file formats and network protocols. Mac OS X is not wide-open like Linux, but it is more accessable than Windows in that respect.
For government operations, I'd suggest Linux servers and Mac OS X for desktop computers. Public sector productivity would be measurably improved with that arrangement.
One other bit of news - H-P is about to buy EDS.