Missing the point
I agree with Psymon above, and also think many of the responses miss the point
Linux cannot become a dominant desktop until such time as it develops a way of allowing windows based applications to be installed and run seamlessly. And by that I don't mean "install wine and then tweak this and then run that" I mean put in a cd and click the nice little flash INSTALL ME prompt. This then needs to trigger some underlying handling to translate the OS into whatever the app expects and let the program *just run*. And this needs to be done out of the box when a user first gets the machine, not as an aftermarket tweak.
Lets face it. Windows has the dominant market share for desktops, so programs are written for it. If the *nix system cannot run them by default, it has failed in its goal.
Yes, alternatives are available, but the user should not have to use them, especialy if they already own the software.
And don't just dismiss specific programs as *niche markets*. Niches can be pretty damn large. Two examples. Banking uses Excel heavily to do all sorts of things that Microsoft never dreamed of. I've seen Excel running as essentially a full trading suite, with live data being updated simultaneously by multiple users in a 24/7 environment. How that was done I have no idea, and my support consisted of crapping myself and praying a reboot fixed it, but I strongly doubt it can be done in Open Office. There are a lot of traders around the world, and I'd say all use Reuters data tied with Excel. Sure, Reuters could develop a plugin for open office, but do YOU want to pay them to do it?
A second niche, a very large school a friend teaches at has a big piece of software that handles student data - reports, exam results, attendance etc. The software is written as a desktop suite, and runs on windows 95 98 NT4 and 2000. It doesn't run properly on XP, but then none of the local pcs run XP either. This is the fourth software suite they've tried, each headmaster likes to bring the one from his old school in as an "upgrade". All essentially work the same however. Find me a linux based alternative to that, as there are a lot of educational facilities around, and they all need something similar. Niche yes, small no.
Finally, many people above were talking about embedded or rebadged operating systems, and servers. Frankly, I think most people would be hard pressed to say what OS is running their embedded device, and I've seen windows errors crop up in everything from ATMs to airport boards. I've also seen unix errors on the same devices. Which is more suited is an irrelevant argument - the original article talks about desktop PCs, not routers or atms. As for servers, they are normally specifically built to do a few things well. File servers could be *nix or windows and most users wouldn't know or care - across a network they all seem the same.
Application servers are the same, certainly most web servers these days seem to run apache.
Once again though, its arguing the wrong thing. Linux, or unix, or debian, or ubuntu, or whatever, they are not suitable to replace a desktop for the majority of users oustide of a home environment, with the exception of *niche* roles like software development.
Think of corporate environments. Admin - no time and attendance programs. Accounts - not enough compatibility with regulators. Trading - just no.
Educational environments? For CS students, sure, maybe. For staff, no.
What about military, or geosciences. Any linux based GIS programs?
You name a program that linux provides as an alternative, I can probably find another large *niche* that there isn't an alternative for. *Thats* the point. Windows allowed the creation of all these programs. If you don't support them, you'll never expand outside of the small niche market of the home tinkerer.