If you ever repaired a phone than you will notice that the cameras are all swappable. Sao you could pull them easily out and have a working connection (theoretically). But the problems I see is the typ of possible connection you have. Not the greatest solder points for Hackers because they needed to be as small as possible in your smartphone.
The second thing is the housing. When your camera is used in your smartphone there are mostly small anchors holding your camera in shape. It is pretty hard to build up a new stable housing for this type of cameras.
The last thing is the protocol. I sort of believe that you need special chipsets to decode the video stream or to communicate with them. And speaking of hundreds of millions of cell phones.. There are adequate different types of cameras around too.
But I would be pretty happy to see such a thing working. Maybe we focus on some type of cameras and hack around a bit.. The problem here might be the data sheets of the cameras.
Oh.. and have you thought about the lenses ? Mostly the lens needed for the camera are built into the case of the phone. But that can vary of course!