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.
But seriously, that's why they've GPL'd XFS, and have done a lot of work on the Linux fs layer. They plan to sell CXFS (Clustering XFS) to those who need it.
And rightly so. But, server is a server regardless of how many processors it has. There are many more 4 processor linux servers out there doing real work than there are the big iron boxes. Each has it's place.
Bigger is not always better. Except when it comes to mean jar-heads covering my scrawny USAF hide. Semper Fi.
/john