Using AJAX to interact with MVC
Well there's not much code to review here, but for what's here, I'd say it looks okay. I would only make one suggestion. If all you're going to do in your controller action is return single string wrapped up in a JSON object, why not dispose of the JSON and just return the HTML as content?

Ajax call:

function getPreviewText() {
    $.ajax({
        type: 'POST',
        url: '@Url.Action("PreviewWiki")',
        dataType: 'html',
        data: 'source=' + $('#markItUp').val(),
        success: function (result) {
            $('#previewMode').html(result);
        }
    });
};


Controller action:

[HttpPost]
public ActionResult PreviewWiki(string source) {
    return Content(m_wikiEngine.Render(source, GetRenderers()));
}