Yeah, so this is basically awesome. I've written a LaTeX-based markup language that I'm using for my Ruby on Rails Tutorial book (http://www.railstutorial.org/book), but it's really designed for putting math & physics books on the web as HTML while still making nice PDFs. One big challenge I've faced is making nice HTML math typesetting, which I solved using texvc (the secret of Wikipedia's math typesetting), but unfortunately texvc is no good at inline math. I was not looking forward to solving that problem. Along comes MathJax, and now I don't have to!

N.B. Being able to benefit from unexpected advances like this is exactly why I standardized on LaTeX, even though it's kind of a pain to convert it to HTML. If you're trying to solve the math typesetting problem and not using LaTeX, you're on the wrong (coughMathMLcough) track.