A few comments to various pasts above:
1) having two photo-diodes under the single microlens and color filter of each of 20 million photosites will not give any noise advantage over having a single, larger photodiode at each of the same number of photosites: adding the signal from the two photodiodes more or less reproduces the signal that a single photodiode would give, and the same total photon count means the same shot noise, and so on. Except that having two smaller photodiodes probably wastes some space on extra wiring and circuitry, so reduces well capacity, and likely adds a little "blind area" between the pair of photodiodes where light is not detected. So the overall effect is likely slightly worse DR and QE than an equal number of "normal" photosites. Then only "noise advantage" would be comparing to the per pixel noise measurements of a 40MP sensor of the same size and technology, and hopefully the fallacy of comparing IQ "per pixel" rather than "per image" when comparing sensors of different pixel counts has been thrashed out here often enough.
2) For birding and the like, carrying the bigger, heavier, more expensive, slower frame rate D800 and then cropping from the same focal length to get a bit less reach (15MP in place of the 20MP) sounds like an imperfect substitute! Ignoring all those other factors and addressing only the reach (which is still a bit worse worse) misses the point. Of course, if you already have a D800, you can make do quite adequately for such tasks, but let us not pretend that there is no advantage to using a tool better suited to that task.
3) As to the question of whether we will still need the flipping mirror if this new technology can AF as well as the best SLR AF: I already feel no need for a flipping mirror, but for some, issues like EVF lag and problems in very low light are still reasons for keeping the OVF option. (Not for much longer, I am guessing: the latest Sony EVFs and the new Olympus/Epson external VF4 seem quite impressive.)
P. S. There still seem to be people who expect 35mm format SLR's to end up with roughly the same pixel size as smaller SLR formats, so that they can counter any claimed advantage of the smaller formats through cropping. And yet the recent trend is almost the opposite: Canon and Sony now offer about the same maximum pixel count in APS-C as in their 35mm format cameras, and the D800 is the only exception --- and it still only matches only 16MP in DX crop, so significantly lower resolution than 24MP DX format sensors when used with equal focal lengths and cropping. If anything, there has been a trend of increasing the gap in lp/mm sensor resolution ("pixel density") in the eleven years since the 14MP Kodak 14N and 11MP Canon 1D arrived alongside 6MP APS-C alternatives.