It occurred to me this week as I reread some William Gibson was that all science fiction (including stories of the near future, like Tom Clancy novels) eventually becomes alternate history. That is, the future imagined in 1984 (the year Neuromancer was published) is a future of 1984, if certain things had gone differently. In some ways it tells us more about the 1980s than it does about the 2050s. In this sense, the earliest steampunk stories were the writings of Jules Verne and the other Victorian science fiction authors. This may be the most authentic writing in the subgenre, and is often included among the conventions of the setting. (With the benefit of knowing how actual events came out, though, we can add in a kind of dramatic irony in the technology that isn't present in these works.)
I suspect one reason steampunk is popular is that it is far enough back to set this effect into sharp visibility, but early enough that science fiction had been invented. Earlier eras in a certain sense didn't have an alternate future to follow. The future that the Thomas More and other Utopian authors imagined, for example, only differed from their present politically, not technologically. Perhaps science fiction requires only the existence of science-- Francis Bacon's New Atlantis, for example, is a good candidate for the first work in the genre, written by one of the inventors of the scientific method.
Aaron Diaz turns all this on its head by imagining how future civilizations will view their past (our present.) His time travelers get some things wrong, mixing clothing in inappropriate ways, for example, and getting the slang wrong. The 80's themed cafe in Back to the Future II, which mixes Reagan and Qadafi with Max Headroom, is another example of this. Of course, both of these stories, written in our past, have yet one more joint in their zigzag path through time.