Note: These bots were retired on April 4, 2017 after years of service.
I've already posted about this too much on Twitter, but over the last couple of weeks I've turned out a couple of bots: @ArkhamSingles, @LesserKnownCase, and @CyberSkyMall. The technique is basically the same as the (now mostly broken) Poe-based character generator I wrote a few years ago. (update 2016-06-16: the Poe generator went offline a few years ago.)
The way it works is simple: first, using a part-of-speech tagger, pull all the nouns and adjectives out of a source document. Then, using WordNet, sort the nouns into broad categories, in this case events, locations, items, and words that describe people. Create a fill-in-the-blank sentence with some adjectives in front of nouns of the right category and you've got MadLibs.
The fun thing about the technique is how authors are as distinctive as you'd expect; on top of that, since the method just uses lists of words, it's easy to mix things together so you can get, say, half Indiana Jones and half Lovecraft.
A much easier proposition is putting the pre-compiled list files online so they can be used to make generators; Abulafia makes this easy, and I've gone ahead and added generators for Lovecraft, Leiber, and Dunsany. If there's any other author I should toss up, do tell me.