The Sony Alpha 7 and A7R have just been announced and feature a full frame interchangeable lens mirror-less camera for around $2000. This niche was held so far by the Leica M at a much higher price point. Unlike the Leica, this is not a range finder.
Brian Smith has a field test that show what we can expect: an IQ Sony style (pun intended), ie good.
I believe that for a modern design system, “full frame” is possibly overkill, and Fuji has shown us that you can get top notch IQ with an APS-C sized sensor. Most DSLR systems, like Canon EOS, Nikon, Sony or even Pentax were designed for 135 film, as well as their lens, hence the “full frame” bias. This is also why I considered the RX-1 overpriced and overspeced, even though some people have found it worth it ; and with a top image quality as well. At $2000 body only the A7 can even be cheaper than the RX-1.
But this Sony is an E-mount (like on the Nex), with a short flange distance, that allow an easy adaptation of older lenses via easily available mount adpaters, including M-mount. This is probably why this camera has a lead off the m4/3 or even the Fuji. You get the real deal, the same field of view on your older lenses as they were designed. The only other alternative today is the Leica M type 240, which is both much more expensive (several time the price), hard to come by, and for which adapted lenses require either the use or the rear LCD or the external EVF.
I do believe that the A7 and A7R will find their way in the hands of people that have a large collection of lenses to adapt, and we can be largely confident that the image quality will continue to meet the expectations. Sony has really shown leadership on a market that was dominated by Olympus and Panasonic, and show that Nikon and Canon have to worry given their disappointing incursion into mirror-less land.