I had a dream last night that I was suddenly brought back in time by twenty years, retaining my present memory but reverting to my body of a seven-year-old where I happened to be in 1983 (well, only Zeus remembers what I did on Saturday the 9th of July 1983). I woke up almost immediately, so I can't tell you more about that dream.
But it is an amusing gedankenexperiment. How would I proceed, in 1983, to prove that I had lived the years 1983–2003? If I ended up in the Middle Ages, I could prove that I know lots of science that was then completely unknown (and end up burned at the stake for practicing black magic). But 1983? There have been great technological progresses since then, of course, but nothing that I could prove my knowledge with (I could pass for the inventor of hypertext before Berners-Lee, of course, but that would just sound like a nice idea, not a proof of technological foresight). Any ideas? In theory I should know some mathematics that were not discovered in 1983, but that wouldn't prove much except that I'm a precocious kid (and so I would be, I guess), and anyway there would be missing details (there isn't much of which I could give a full proof that wasn't known in '83, and I would have a hard time convincing experts).
Nor could I prove my claims by predicting the future: any small change made to the present, like my knowledge being different from what it was in the 1983 that I really lived, would change the entire future course of the Universe (mumble butterfly effect mumble). The near future would be very similar, of course, but all sorts of little coincidences would take place differently and in the end my knowledge would be useless: I would be stuck with the memory of a never-never time, the ghost twenty years of a parallel world, an alternate future—and probably with a sense of loss. Some happy coincidences which have brought me joy in the future I “remembered” would perhaps not take place, and I would be disappointed, even though some misfortunes could probably, on the other hand, be avoided.
Pondering over this idea, I concluded that, on the whole, I
wouldn't like it to happen. I don't believe I've made many mistakes
in my life that I'd that desperately want to be given a chance to set
straight again; I sometimes think that I've wasted my time at various
periods of my life that I could have used more productively, but not
so much that I'd be willing to go through the pain of living throught
it all again: basically, I'm mostly satisfied that the years behind me
are just that, behind me, even if I sometimes wish some
things had gone differently. And I guess I just couldn't stand
waiting another ten years for the World Wide Web to
appear. 
Besides, as I just mentioned, there would
be the frustration of having lived through a period that never
existed, some years mysteriously vanished from the history of the
world; the feeling of having lost my home (Universe). Not counting
the frustration of not having paid more attention to what went on in
1983 so that I could predict at least some things accurately; or, for
that matter, of not knowing or remembering enough of what could turn
out to be useful. So if some mischievous daemon offered me the
possibility, I would decline it, I think.
If you find the idea amusing, you can also read my crazy speculations about the mathematics of time travel (in French).
Interestingly, they had a story on the news tonight about a guy (Terry Wallis) who regained consciousness after nineteen years in coma. (They make it sound as though it were cause for happiness, but I guess this guy's brain is now total havoc, and he'll probably never speak again, so I'm not sure he's in an enviable situation, not to mention the fact that he totally lost the best years of his life. A real cause for rejoicing would be someone waking up from prolonged coma with no ill side-effects whatsoever.)