Unix boxes can be secured to C2 level. The spec is called the Orange Book by the people that do that kind of thing for a living.
/john
Something that can compete with high-end IBM S390 mainframes in a corporate/government environment. I'm picturing a Sun Fire 15k (>US$4,000,000)with 106 UltraSPARC III processors.
If you worked as a network admin for a company that had one of those, and proposed the bright idea of loading Linux on it, the IS&T chief would probably fire you on the spot.
Any computer that a man can pick up alone and move from one side of the lab to the other is but a toy.
Before you mention that IBM is also embracing 'Tux' the Penguin, you might also want to notice that the IBM AS/400 midrange does most jobs lots better, stabler, and cheaper than a similarly-configured Linux system.