A basic implementation is there, but it's not yet fully bulletproof (contributions welcome!).
There are a couple of known issues surrounding blockquotes in lists (and perhaps vice versa) — #2; and also how to handle HTML elements outside of the markdown subset (keep them/strip them?) — #3.
to-markdown can also be run under Node.js. Install it via NPM, and then:
var toMarkdown = require('to-markdown').toMarkdown; console.log(toMarkdown('<b>Hello world</b>')); // Will output '**Hello world**'
Node.js unit tests can be run with NodeUnit:
$ npm test
This will test both server-side and client-side tests (using zombie-qunit).
to-markdown is copyright © 2011 Dom Christie and released under the MIT license.