While I'll admit it was several years ago, Walmart sold a line of PCs w/ Ubuntu pre-installed. Changing OSes is no small task. Little things you do on a daily basis are slightly different, and it can be a real hassle, or waste of time, while you are trained for the differences. I think that having Linux available (a quality distro that is) as a preinstalled option on the shelves of big box retailers would certainly increase adoption, but only if the support was there from the OEM. I'll go further to say that it will require a good rolling release distro. The average end user isn't going to put up w/ reinstalling every 6 mos or suffering quickly ancient software because they didn't.