Nerve impulses race down the spine, branch off at the shoulder and travel down the stump of an arm. Electronic sensors pick up the signal, and convey the impulse along wires to the mechanical hand, allowing the amputee father to give the "thumbs up" to his Little League son who has just caught the third out.
Such technology exists today, though constantly undergoing research, development, and improvement, to integrate circuits with the human nervous system. Beyond mere replacement parts (like Teflon knees), mechanical parts that can integrate with the human body offer many potential future options. While mechanical add-ons or replacements might certainly be modeled on normal functional parts, there's no reason that a person could not take a part that served a specialized function. Perhaps a window washer would opt for extensible legs (reminiscent of Inspector Gadget, perhaps). A handyman might "upgrade" to a multi-tool hand, with the ability to summon screwdriver, hammer, wrench or flashlight without the need to rummage in his toolkit. An overworked executive could even "install" that second pair of hands she always said she needed.
Similarly, as the interface between nerves and circuits is fine tuned, the idea to hook computer applications up to the senses, or even the brain itself, becomes an option. News feeds or stock prices could be fed directly to the optic nerve. A radio link to the internet could be maintained, allowing eye movements, or even pure thought, to navigate and locate that puzzling song lyric, and then to send a message off to a friend. Math co-processors might be installed, allowing everyone to be good at algebra and calculus. Inventors might connect to recording devices so that they'd have a record of their thought processes (for the patent office).
In the long term, one could imagine that people would be as customizable as a new car or a personal computer. There would be ethical and legal concerns. Who decides what implants a person gets? (Government, parents, or individual? If the person, at what age may they begin to choose?) Are special laws needed to regulate implants? Do half-man, half-machine cyborgs have the same rights as regular people? What if they are only one tenth machine? Such questions make for interesting consideration, but are still a concern of the future.
A fear shared by some people is that implementing machine parts somehow dehumanizes the person. If machines are cold and calculating, would the person then become cold an calculating as well? If the brain were replaced by a computer, then perhaps this is likely. If, however, chips are implanted to assist the normal brain's functions or capacity, one would think that personality would be unaffected, save perhaps by the psychological effect of having increased abilities. This is still beyond current technology though, so future vigilance will have to provide an answer in time. Religious concerns focus on whether the person would still have a soul. Since the soul is a part of the body, the reasoning goes, replacing parts might also remove the soul. Souls are, of course, poorly defined and intangible, but since people with amputations or replacement organs are still believed to have souls, it stands to reason that a cyborg would retain their original soul as well.
The future potential for human adaptation through technology is difficult to define, but a glance into works of science fiction gives a good idea of how much can already be conceived. "Stronger, faster, smarter" tend to be the traditional themes, with the ability to hook into networks newer, but also common. It seems certain that all these options will eventually be available to the common man, but when is less clear. Odds are that when it arrives in force, it will have large cultural impacts, and take a few years for society to adjust. Then again, everyone is so used to seeing cell phones attached to ears, perhaps the advent of cyborgs won't even raise an eyebrow.