
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.
I love Sun, mostly because of the things they've given the computing world (Java, NFS, etc, ad nauseam). However, Linux can do one thing Solaris can't: run at its full potential on x86 hardware. This is why Sun is losing market share; *SPARC machines are slowly being irrelevanted.