I wrote a comment on David Thomas' blog a while ago. (If you happen to be my father, please do not be offended.)
I think that it's very difficult for "normal" (not technical minded) users to switch to Linux. Even if they were able to evaluate whether they could use Linux in a productive way, migrating personal data and installing an operating system are likely to be tasks that are out of reach for many users and that they are afraid to do. This fear is not necessarily a bad thing. It might shield them from a lot of frustration caused by missing hardware support (mobile phones, printers) or in the worst case data loss. Many people I know wouldn't dream of reinstalling an OS.
Take my father as an example. He spends a lot of time at the computer and he knows his way around. He would reinstall his OS. But he still sends me links to Windows software though I told him a dozen times that I'm running Linux and that my OS is (generally) unable to execute .exe files. A while ago, I also had a difficult time trying to convince him to stop using the AOL software though it is unnecessary and doesn't allow to export e-mail. Software lock-in is a concept he doesn't want to think about. The same goes for most other abstract issues that made us happy Linux users. You don't need to care if you just want to create a birthday invitation in MS Word or something like that.
Of course, you can do that in OO Writer under Ubuntu, too. This is true for most tasks. So I guess we could install a Linux distribution on my father's PC without leaving him out in the cold. But I honestly doubt that it will be beneficial to him. Besides, I would feel obliged to help him find software and to explain everything that he doesn't understand (in case I do). As long as he’s using Windows, I don't have that responsibility. That's why I partially think it's the best arrangement for both of us.
Sorry, I don't have a real conclusion for this comment. This is just something that bothered me for the last few days.
 FUD disclaimer: This is not Linux' fault. I know that. But it’s an issue nevertheless. Ask my Nokia.
A possible conclusion might be: Do not install an operating system onto other people's computers just because you think it would be a good idea.