//Also, I'm unsurprised that HTML5 developers are well-represented at hackathons,//

The pattern I observed in the internal hackathons, of the company I work for, was the dominance of web developers participating/winning over c++/Java developers.

As you said, the main reason for this was web developers were able to produce something that the audience can 'see' and in short amount of time.

Also, I've been thinking a lot about the survival of C++/Java developers in web company - because of the introduction of technologies, like Node.js. Because, now it is possible that a JavaScript programmer could do both back-end and front-end stuffs.