One of the things that bugged me about the movie "Avatar" (I know, I know) was the idea that 150 years in the future, someone who had suffered a spinal injury would be rolling around in a wheelchair, and getting him new legs would be so expensive that only some people would be able to afford it. I realize the protagonist's inability to walk on his own was central to the plot, but none of the other technology in the film required that it take place that far in the future; they could just as easily have said it was 2054 instead of 2154 and it would have been much more plausible.
Why do I bring up this bit of nerd nitpickery (nerdpickery?), you ask? Because here, via Popular Science, is a man controlling the movement of his prosthetic arms with his mind, the first double-amputee to do so:
This research and development is funded with your tax dollars, which is pretty cool. So how long will it be before prosthetic limbs can move with all the responsiveness, precision, and dexterity of real limbs? Who knows, but it's obvious that the technology is going to keep improving. They're even developing smart skin for prosthetics to give the kind of feedback you get from your own skin.
So here's my question. Let's say it's a few decades from now, and prosthetics have improved greatly. They go into large production, since there are people all over the world who need new limbs. When do we cross the point where people start saying, "You know what? I'd actually rather have that than my own limb"?
Let's imagine, for instance, a soldier who think that prosthetic legs that are faster, stronger, and tireless would actually be more useful than ordinary legs. What happens when she volunteers to have her legs removed and replaced with prosthetics? Your reaction might be, well that sounds crazy. Maybe. But legs are big—they make up about half your body. What if we develop prosthetic eyes with superior vision than your eyes—maybe throw in infrared, some zooming capability—would you be willing to replace one of your regular eyes with the cyborg eye? It's just a little thing, and it could do so much for you.
There will be a point when we realize that these kinds of technologies can be used not just for correction—restoring people with injuries to full capability—but for enhancement, moving beyond what we're now capable of. Many people will resist it mightily. But there are also going to be millions who line up to get modified. Just you wait.
(postscript: If this movie sucks, the 8-year-old boy I once was—the one who had a "Six Million Dollar Man" lunchbox—is going to be seriously pissed off.)