
Linux only has a small percentage of the computing market, however Microsoft already considers it a major competition as the open source OS steals the hearts of many users. Following the hard numbers though, Microsoft also increases its market share on both server and desktop space with time. The only logical explanation is that Linux steals quite a market share from the traditional UNIX providers (SCO, Sun, SGI, HP, IBM). But only Sun seems to truly be in a real Linux trouble, as it is the one with a resistance to Linux integration to its full product range.
MS Windows makes the worst server of all major operating systems, however many use it. Why is this? It's because people use Windows at home. Using it at home makes people more comfortable with the software, so they want to use it at work as well. They want to use it on the server and workstation because it's what they know how to use, reguardless of whether it's the best choice. It seems pretty obvious that many Linux users work in IT fields, and so these people bring Linux to work the same way Windows users bring Windows.
UNIX will continue to lose ground, not because it's a worse OS, but because people feel more comfortable using Linux. Linux is what they know because they use it at home. Nobody uses AIX, HP-UX, or Unixware at home because they are too expensive for home use and usually run on expensive hardware. And possibly more important, there is less "home" software, such as games like Quake 3 and Return to Castle Wolfenstein, which is availble for Linux.
UNIX will probably continue to lose ground on the server, even if it is better, and this is why. Linux is popular on the server for the same reason Windows is--people are more comfortable with it because they know it from using it at home. UNIX is probably going to lose because of this, be it to Linux, Windows, or a combination of the two.