Back in the earliest days of PCs, Steve, people didn't buy any particular hardware or OS, instead they bought a machine which would run VisiCalc. In many respects, that's always been the way since, people have bought a machine to do a particular job. At one time, Macs dominated the reprographics world, because they had a better GUI and, most importantly, they handled colour correctly. MS finally woke up to that and, once Windows had a workable colour management system, it was as common to see a Windows machine as a Mac.
I think the problem for *nix is, and will remain, the lack of big name software. Until, for example, Adobe produce a version of Photoshop which will run on Linux, I will need to maintain at least one machine which runs either Windows or Mac OS.
What I can see happening, though, is that my laptops and netbook will move to Linux, handling day to day comms and 'office' functions, leaving the desktop to become the reprographics machine.