Happy reveal day! It is I, your author.
I am Andrew Plotkin, fairly-well-known interactive fiction person. (I saw at least one reader guess that.) I am delighted and humbled by the outpouring of comments here. I had fun writing this thing, but I had no idea it would be so popular! I cannot hope to answer every comment. Thank you all for enjoying the story!
I wrote BTYT in Inform 7 (inform7.com), a popular IF design system. However, I customized the library quite a bit. A lot, in fact. As several people noted, BTYT is a hybrid -- mostly a CYOA-style game, but with an IF-like inventory. This allows me to break away from the standard CYOA "select from these three choices" model.
(I could get into a whole tangent about whether this is really "interactive fiction" and what that means and whether old-style (parser) IF is insufficiently approachable and whether a different kind of game would have worked better in Yuletide, or not as well... As you might guess, this is a hot-button issue for me. So I will skip that for now.)
I have moved the game from its original anonymous Dropbox URL to my home web site. I have also updated the game slightly (put in my name, put in credits, fixed the no-cookies bug). The source code is also now available, if anybody wants a look. It is *not* so tidily organized that anybody can start writing a game "like this" -- I apologize for that. But if you want to tackle such a project, let me know and I can try to clean it up.
I hope this game has served not just as a Yuletide treat, but as an invitation! It would be awesome to see more interactive fanfic next year. I know there were at least two other interactive works *this* year -- Flourish did a Twin Peaks visual-novel piece, and genericgeekgirl wrote a small House Hippo game in Inform 7. (Blame Flourish for getting us all into Yuletide in the first place.) More of these! I promise it is less work than writing some of the (fantastic) long-form Yuletide stories I've read this past week.
Assorted other trivia:
Back at signup time, I wrote down a list of fandoms I could imagine writing IF for. (Invisible Cities, Snarkout Boys, Codex Seraphinianus...) I particularly liked the idea of Marco Polo visiting Seraphinianus! Crossover made in... somewhere, right? But the assignment came in for xkcd, and then Marco Polo just -- stuck around. I think I literally forgot which crossover I wanted to do, and made this one work.
Much later, *after* I'd designed the whole thing, I remembered that someone in the xkcd strip *is playing Marco Polo*. The game, I mean. (http://imgs.xkcd.com/clickdrag/1n11w.png) I would feel dead clever if I'd *remembered* that when I stuck in the reference, but no, total coincidence...
I have met Randall Munroe, as it happens. We were on an ad-hoc panel discussion together last summer. So I sat next to him and we shook hands and tried to talk semi-coherently about Kickstarter and being an indie artist, but we didn't have a conversation as such. I did not contact him about BTYT, and I haven't gone looking to see if he's mentioned it.
(Hm. It would make a good interactive illustrated iPad comic, wouldn't it? Have to think about that.)