Considering the fate of national retailers like Circuit City, I too do not think this is the model to follow. The costs of the storefront, plus the retail/commission model used by the big box stores would not work for Linux.
Specialty stores, the "hole in the wall" places might. These are the small stores that generally offer the more high-end components and custom build systems, to the more discerning customer. Because of this, they would be better suited to sell Ubuntu or other Linux systems. However, they are also niche-fillers, and do not offer the marketing muscle of a national chain like Best Buy or Future Shop. And, a large percentage of them already are well familiar with Linux. That's how I got started on Linux; I was introduced to it by another customer at one such place I used to frequent, on the laptop he'd brought with him.
So, I definitely agree that if there was a mechanism to get more non-Linux users to interact with a Linux box, it would explode the market. Consider the poor girl who ordered a computer from Dell, but did not pay attention to the fact that she was ordering one with Ubuntu installed, about which she knew nothing. Had she encountered Ubuntu at a storefront, she may well have had an entirely different (and positive) experience, and may have even converted.
Last week, I had my chance to evangelize. I was traveling through Heathrow Airport in London with my System76 Serval Pro. I was at a cafe in Terminal 1, playing around with some music files on my attached mp3 player, and noticed this guy sitting next to me was looking at my system intently. I showed him my Serval - a beautiful machine with its glorious WUXGA display - and my Ubuntu layout, customized with Mac4Lin. I demonstrated its ease of use and power. He'd never heard of either, but was suitably impressed enough to express interest in looking into Linux further. I even gave him the bootable DVD from the Linux Format mag I'd just bought, so he could try it out on his own system.
In the perfect world, people could go to a Best Buy or some other national chain, touch and play with a Linux-loaded computer, and see for themselves the FOSS way. I bet a huge number would, in comparing a Linux distro side by side with a Vista or even Mac system, would choose Linux. Even if they didn't, they would at least know what Linux is, and that would also be good.