Interesting. After looking at the MXHR more, it appears as though it was adapted from Digg.com[1] (aka DUI.Stream[2]), and then later adapted by Facebook[3].
Yours sounds more elegant in that it can handle JSON naturally, but I wonder if the other might be better suited for binary content (not sure in which context that would make sense, if any).
Either way, I find them all fascinating, and I've starred your project :) Will keep an eye on it.
1. http://techfoolery.com/mxhr/mxhr.js