Comment 173 for bug 747197

Well, the example also does that. As does this: http://code.google.com/p/phantomjs/wiki/QuickStart#DOM_Manipulation but it might not be so clear.
This is the interesting bit:
var page = require('webpage').create(),
    url = 'http://lite.yelp.com/search?find_desc=pizza&find_loc=94040&find_submit=Search';

page.open(url, function (status) {
    if (status !== 'success') {
        console.log('Unable to access network');
    } else {
        var html = page.evaluate(function() {
            return document.innerHTML; // <- get all generated and styled HTML
        });
        console.log(html);
    }
    phantom.exit();
});
There is a similar project for Gecko called offscreen but I can't see how it's possible to get the generated HTML with this though.
Project Page: http://offscreengecko.sourceforge.net/
Source code: http://hg.mozilla.org/incubator/offscreen/