That said, I think that in the longer term, the days of reflex mirrors are numbered. Within 5 years it would not surprise me if most "single lens cameras" have done away with the reflex mirror. The biggest stumbling block may still be a decent EVF.The biggest stumbling bock should be a good fast AF. We may say that The New Nikon mirrorless has both contrast detector and phrase detector built into the sensor(??). But Nikon may be holding the patent right on it. So it will be off limit to other manufacturer. If we are using mirrorless, we can just use the LCD screen at teh back of the camera as view finder. All point and shoot user are doing it. we do not need EVF and use the mirrorless like a DSLR.
On the other hand, Canon can build a copy of the M9 with Leica M mount and made a new line of lenses. Canon is fully capable of doing it. Canon has been making Leica copies for decaded until the 60's. The patent right of M mount has expired afew years back.
The biggest drawback will IMHO be using an EVF, the current generation are still IMHO pretty poor looking and I have my doubts they'll ever look great given the size limations.
The NEX seems to show that mirrorless ultimately does not have THAT big an impact on lens size and most serious photogra0hers are not going to want a grossly unbalanced body,. If the body is held back my having to balance the lens(and include the current amount of manual controls) and the lens arent that much smaller then is a much smaller reduction in size worth giving up the OVF for?
My feeling is that we'll probabley see the high end market spilt between DSLR's with the current wide range of zooms and large sensor ultra compact mirrorless that use mostly primes.