Sorry to bump this old thread again. It just came up in search and I want to throw in a couple of points.
I agree with Jonathan, that this is not just an issue of default viewport size for the device. The problem IMHO is that you are assuming that the nature of the device is an adequate proxy for its viewing distance, and Jonathan's use case of using a TV as a computer monitor shows that that is not necessarily the case.
Devices can be used at a variety of distances - right now I have a monitor about half a metre in front of me but another one across the room is displaying a dashboard which I can also clearly read. Both are standard computer monitors with a similar resolution, and the knowledge of how far I am away from the screen is hard coded into the application that is displaying content on the screen, not the screen itself. I think expecting devices to present viewport sizes that are scaled to the 'expected' viewing distance is likely to never happen.
Second, the choice of layout that a developer would make for a small viewport will depend on whether that small viewport is a physically large screen that is far away, or a physically small screen that is close up. At the FT, this distinction is exemplified by our Next FT mobile/responsive site vs our Big FT billboard site. Same virtual viewport size, different layout.
This distinction could potentially be explained by the fact that we are bundling other media property assumptions into these products too: interactive vs non-interactive, for example (which certainly correlates, although not perfectly, with viewing distance).