This. This is it- so long as Linux doesn't come in a form that the majority of casual home users can just start up and use, it won't be a successful mass market product.
Well, they can
now. Ubuntu is only one of many distros that are fairly easy to use, just as easy as Windows or Max OS X, and has been up there for several years already. There is a bit of a learning curve if you're used to Windows or Mac OS X and haven't used anything else. However, if this is the first computer interface you've ever used, the learning curve is going to be just as much for Windows, Mac and Linux.
The problem is that the computer manufacturers are getting benefits out of the commercial marketing schemes of Windows and Mac. If they aren't outright paid to sell machines with those OSes on, they get most of their business from people who see Windows and Mac commercials and want one of those or the other.
A POD would require someone, somewhere back there, choosing to spend money on marketing. Some major corporation which decides to sell inexpensive, but high-quality computers by releasing its own, really user-friendly distro of Linux. At the same time, it's funding the development of this distro, which only improves Linux in general.
An alternative POD, however, would change the PC market completely. Back in the very early '80s, before DOS and Apple really began to take off, you could easily contemplate a world where the OS wasn't important, it was the manufacturer.
The idea of buying a machine because of a specific interface is not exactly ubiquitous in commercial devices. You don't buy your washing machine or oven based on the color of the dials.
Most people don't buy their cars because of the location of the gear shift, the number and appearance of instrument readings, or where the seat controls are. You might prefer a car based on some of these factors, but you wouldn't turn down a car with better performance/cargospace/features just because their dashboard wasn't specified and designed by Car Interface, Inc.
If a car analogy isn't good enough, look at cell phones. Each device, sometimes even with the same brand, has a different interface. Whenever you get a new one, you need to learn how to do all the things you want to do with your phone all over again. However, people choose phones based on features, or whatever discounts the service providers are currently operating.
So, if early PC manufacturers built more in-house operating systems, or contracted out their operating systems to companies that were willing to reprogram them according to the manufacturers required specs, then operating system wouldn't be as important as manufacturer.
Of course, this isn't the way computer manufacturing and operating system development works.