if by modular you mean Nikon can redesign and change the internal components without changing the shell and then sell it as a new model, that has been what Nikon, as well as others, has been doing for so long, as can be seen in the list I mentioned in an earlier post, post
#15...
if by modular you mean a consumer can go back to Nikon to get them to upgrade the sensor, then there is no evidence at the current moment that Nikon is going in that direction with the D3X... they could do it if they wanted to, and they could have done it in the past, just like any other company could also have done it in the past, its just that no company (other than Red) is showing signs of going down that path...
so in answer to the statement "Is Nikon D3x the start of modular system .." as stated in the title of this thread, my answer for now is "no"... if there is evidence to show that the answer is "yes", I don't believe I have seen it yet, but do believe me when I say if it is "yes" I would be happy for that fact