I've always figured that large displays were never produced because they are fragile, and making the case strong enough to protect the display makes the total package too heavy to use. With this technology, not only is the display lighter, but the case can be as well, since it doesn't have to be so rigid. Since the current smaller readers aren't particularly fragile or heavy, I'm not surprized that this tech is being introduced where it's needed most.
I have a 9.7" reader (1200x800), and although the screen is pre-Pearl technology, it's useable for reading typical PDFs when in landscape mode. This new screen is effectively just two of my screens put together (1200x1600), so I think it would be at least as readable in portrait mode as mine is in landscape. If the contrast is closer to that of Pearl displays (like in the Onyx M92), then I'd say that 150dpi is good enough for reading PDFs in portrait mode on such a large screen. I hope that they keep improving the pixel density, but this is a good first step.