There's an argument currently raging about whether or not a PC should even ship with an operating system of any kind preloaded. Would the lack of a preloaded OS, be it Windows or Linux or what-have-you, level the OS playing field that much more?
I'd like to think it would. But it really comes down to two issues: 1) what's really trying to be accomplished here, and 2) how they go about doing it.
The first issue seems obvious: give people that much more choice in terms of what OS they run on their PC by default. Removing a preloaded OS from the PC would force folks to at least think a little about the next step -- use Windows, or use something else. I don't know if that would happen, to be honest, since there is still the prevailing assumption among most nontechnical people that the PC is Windows. But again, not having Windows as the default would hammer home the point that it's just one of many possibilities.
The second issue isn't as obvious, because there's a few different ways the unbundling could be implemented. You could have Windows supplied (at an optional, additional cost) on an OEM installation CD or an image-restore disc, depending on the whim of the manufacturer. A Linux distribution also could be provided to the user on CD, presumably at much lower cost. The user could then install what they wanted to install as the first step of setup.
I suspect that if unbundling is offered from the major PC makers, and they're not forced to debundle completely by default, it'll be offered in a way that only makes it one of a set of choices. For example, you can buy this PC with Windows preloaded, with Linux preloaded, with nothing preloaded and an install disc of your choice, or with nothing and no preinstall discs -- each at their own price point. If nothing else, this approach would certainly make clear how much the cost of Windows adds to any given PC, even if it's not a lot of money -- and emphasize that other options do exist.
So in short, I'm tentatively for this idea, but it depends on how it's rolled out -- and we shouldn't be unrealistic in how much change takes place because of it.