We are reaching the point where the evolution of sensor design is plateauing and we are seeing only minimal differences between different platforms. And I think this is a good thing because it forces us to return to the original question of how we choose cameras. This article was stimulated by people finding that Leica R9/DMR was taking photographs that were only slightly better in quality than the much less expensive Canon EOS 20D. The Nikon D2X photographs also rival those taken with the Canon EOS 1Ds Mark II even though the sensor size is much different.
At this point in time, you could take equally stellar photographs with the following cameras: Nikon D2X, Canon EOS 20D, Canon EOS 1D(s) Mark II, Leica R9/DMR, Minolta alpha-7 digital, etc. I would suggest that most people would not be able to tell you which body was used with which in 13"x19" (Super A3) sizes except for the different lenses used. I.e., it's lens quality and not sensor quality that is becoming the limiting factor.
Ten years ago, we used to chose film bodies either because of the compatibility of the glass we put on it, or because of the body features, and not because one body takes better photographs than another. In digital terms, the sensor (the film) was the same across all 35mm cameras. In other words, a Canon EOS Rebel with the same glass could take an identically good photograph as a Canon EOS 1v because it shared the same sensor (35mm film). We chose the EOS 1v back then because of its superior environmental sealing, the eyepiece shutter, build quality, reliability, etc.
I predict we'll start to begin choosing our digital bodies based on factors other than sensor density and image quality. The reason you might choose the R9/DMR over the EOS-20D will become the same reason you originally chose the R8 over the EOS-5 ten years ago -- not because of the superior "film" since the film sensor was the same, but because the ergonomics and other mechanics were better.
In this brave new world of digital photography, if we want to upgrade to an entirely new grade of sensor, we'll use medium format digital sensors, which I've blogged about previously.