I’ve completely rewritten my write maths, see maths library to be a little jQuery plugin that attaches itself to editable areas on pages, like
contenteditable elements, textareas, and input boxes. When your cursor is inside some LaTeX, a little preview box appears just above it with the LaTeX rendered through MathJax. I’ve made a demo page on GitHub, and the code itself is available there too. It also works in TinyMCE, if you’re into that sort of thing.
The first I thing I did with it was to write a WordPress plugin which applies the plugin to the comment boxes underneath posts (source code). I’ve installed it on this site and The Aperiodical, so you can use LaTeX with confidence, knowing that it’ll appear how you want on the page. Please try it in the comments box below!