If you're using intensedebate comments on a page that already includes mathjax, this plugin script (to enter here: (http://intensedebate.com/pluginEditor/) ) will use the ambient mathjax to typeset maths in the comments. I have this running in my tumblr, http://jessecmckeown.tumblr.com .

IDMathJax.js

var id_math_stuff = {
    // id_math_stuff.theMJ
    theMJ : window.MathJax,
    
    // id_math_stuff.idMathRun =
    idMathRun : function () {
        if (id_math_stuff.theMJ) {
            id_math_stuff.theMJ.Hub.Queue(["Typeset", id_math_stuff.theMJ.Hub, "idc-cover"]);
        } else {
            console.log("there is still no mathjax?");
        }
    },

    // id_math_stuff.saveEscapes =
    saveEscapes : function (commentext) {
        var the_text = commentext.replace(/\?/ig, "?a");
        the_text = the_text.replace(/\x5c/ig, "?b");
        return the_text;
    },

    // id_math_stuff.escapeEscapes =
    escapeEscapes : function (argRec) {
        var the_text = argRec.text;
        the_text = the_text.replace(/\?b/ig, "\\");
        the_text = the_text.replace(/\?a/ig, "?");
        return the_text;
    },

    // id_math_stuff.switchEdit = 
    switchEdit : function () {
        if (!id_math_stuff.editComment) {
            id_math_stuff.editComment = window.IDeditComment;
            window.IDeditComment = function (commentid) {
                var elt, nowJax, delim, t;
                /** because it only seems sensible */
                if (!commentObj.curUser.userid || commentObj.curUser.userid <= 0) {
                    return;
                }

                /** There shouldn't be scripts that aren't mathjax's! */
                elt = document.getElementById("IDComment-CommentText" + commentid).getElementsByTagName("script");

                while ( elt[0] )  {
                    t = elt [0] ; 
                    nowJax = id_math_stuff.theMJ.Hub.getJaxFor(t);
                    delim = /display/.test(t.type) ? "$$" : "$";
                    t.insertAdjacentHTML('afterend', delim + nowJax.originalText + delim);
                    nowJax.Remove();
                    t.parentNode.removeChild(t);
                }

                id_math_stuff.editComment(commentid);
            };
        }

        if (!id_math_stuff.saveComment) {
            id_math_stuff.saveComment = window.IDsaveComment;
            window.IDsaveComment = function (commentid) {
                var textare = document.getElementById( "IDEditCommentTextArea"+commentid);
                var theString = id_math_stuff.saveEscapes(textare.value);
                textare.value = theString;
                id_math_stuff.saveComment(commentid);
                var ctext = document.getElementById( "IDComment-CommentText" + commentid);
                ctext.innerHTML = id_math_stuff.escapeEscapes( ctext.innerHTML );
                id_math_stuff.theMJ.Hub.Queue(["Typeset", id_math_stuff.theMJ.Hub, "idc-cover"]);
            }
        }
    }
};

id_add_action('comment_post', id_math_stuff.idMathRun, 45);
id_add_action('idcomments_func_load', id_math_stuff.switchEdit, 99);
id_add_filter('pre_comment_text', id_math_stuff.saveEscapes, 2);
id_add_filter('comment_text_load', id_math_stuff.escapeEscapes, 40);

now v 7.1 ! MathJax kindly leaves "display" in the script "type" HTML attribute, whatever OutputJax one is using.