"Well, everyone knows Ripley died on Fiornia-161. What this ALIEN movie presupposes is... maybe she didn't?"
I have a long, long "Exposition" essay up at The Dissolve today inspired by (uncertain) reports that District 9 director Neill Blomkamp's upcoming Alien movie may be a ret-con scenario that undoes the events of 1992's Alien-little-three, or Alien Cubed – anyway, the one where Ripley died. The piece is about retconning in fiction in general, and why it doesn't much impair my ability or inclination to suspend my disbelief at all.
If you're quite comfortable in your chair, and you're stout of heart and nerdy of temperament... Onward!