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.
MathML allows math to be rendered to speech for people with disabilities, something that LaTeX won't do. It is also a much more solid REPRESENTATION. It is not an input language like LaTeX. Think of LaTeX as YOUR favorite math UI, whereas MathML is an underlying representation. The fact that you can View Source and read it is immaterial.
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.