Will Sony release an adapter for the NEX cameras that includes a mirror and prism? Perhaps, but I doubt it. If they do, it will be a stop-gap to a future of completely mirrorless designs. I also doubt that they are planning a modular design. Let me explain.
The market for modular cameras is very small, even in the pro community. Sure, lots of people think that they want one, but they don't. There would be, necessarily, extra cost and size associated with a modular design. It's one of the many reasons why RED ditched their Scarlet idea after so much already invested.
Small size will always be a virtue for cameras. Basically, if they can get smaller, they do. Would a modular Nikon D3x be unbelievably amazing? Probably, yes. But the D3x is already huge. Modularity is the dream of those who haven't lugged around a mega-SLR with telephone lens for five hours. It hurts.
Modularity will succeed in professional video... because it already has. It's been succeeding for eighty years. And again, there, it succeeds best with top pros. The smaller the budget and requirements, the further toward unibody designs the market shifts.
Even though I am a huge advocate of mirrorless cameras, I do not see a future without mirror's and prisms in the best still cameras. I've used all of the best-focusing mirrorless cameras, and also a Canon 5D Mark II and 20D. I like macro photography of bugs, which requires lots of high-speed focusing and refocusing. The mirrorless cameras, be they Panasonic or Olympus, can not even BEGIN to keep up.
Because of that reality, I doubt that Sony will ever go fully mirrorless. Sony has had a great deal of success in the point-&-shoot market, and smaller success in the enthusiast market. It's in the pro and semi-pro market where their sales have been terrible. That's really funny because Sony manufacturers the sensors for both Pentax and Nikon.
Sony wants pros, and that means top-end-unibody cameras with mirrors.