Thanks for the explanation, Bob! I've heard some Russians explain constructions such as пойти в солдаты as being a rare example of using a preposition with the "nominative case," but it makes more sense that it would be an "old accusative." (I've seen this question come up in discussions about "How many noun cases does Russian REALLY have?" -- with some people claiming that there are something like 5-10 "rarely used cases" in addition to the six that are taught in standard grammar textbooks.)

PS. For Gottimhimmel, here's a link to an old MR post about the so-called extra cases in Russian. Keep in mind that this is "just for fun" linguistics trivia and you shouldn't waste time trying to learn these -- for all practical purposes, Russian has six cases and that's that!

But for what it's worth, the в + acc. pl. construction is sometimes described by scholars as the Включительный ("inclusive") or Превратительный ("transformative") case. It's better, however, to think of it as simply as a "special accusative," like Bob said.