There are a number of patents from many camera companies for interchangeable sensors, but so far, the idea has been impractical simply because new technology requires additional connections and electronic speeds that were not foreseen two or three years earlier, and getting tied to out of date technology isn't something engineers want to do.
Right now, many sensors are switching to 8 channel readouts, what will be have in 3 years? 12, 14, or 16 channel, and what speeds? The motherboard and processor tie you down to todays technology pretty well. It also ties down any improvements in autofocus, video, or nr among other things.
Its not really such a good idea when you think about how much it shackles you to obsolete technology..
Its hard to believe that we will see practical interchangeable sensors unless its just for easy replacement of a damaged part.