retweet.js

/**
 * Old School "RT" button hack for #NewTwitter
 * @author http://twitter.com/timwhitlock
 * @version 1
 * @todo deeper integration into twttr.dialogs
 */
(function ( w, d ){
    try {
        var pc = document.getElementById('page-container');
        if( ! pc || ! w.jQuery || ! /twitter\.com$/.test(w.location.host) ){
            throw new Error("Please make sure you're on the new twitter.com and try again");
        }
        pc = $(pc);
        // retweet function; accepts native .tweet element
        function rt( e ){
            try {
                // ensure we have text area for tweeting
                var ta = pc.find('div.tweet-box textarea');
                if( ! ta || ! ta.length ){
                    // fake a click on the home button and come back later
                    $('#global-nav-home').find('a').click();
                    return setTimeout( function(){ rt(e); }, 100 );
                }
                // get original tweet from meta attributes
                var name = $(e).attr('data-screen-name');
                // rip tweet text from .tweet element's innerHTML
                var html = $(e).find('div.tweet-text').text();
                ta.attr('value', 'RT @'+name+' '+html ).focus().change();
                $(d).scrollTop(0);
            }
            catch( e ){
                alert('Error:\n'+e.message);
            }
        }
        // find outer tweet objects in page and bind an
        // event handler that places the old-rt button on
        // mouseover.
        pc.find('div.tweet:not(:has(.old-rt))').live('mouseover', function(e){
            var self = this;
            // add our own button after the reply button
            $('<a href="#" class="retweet-action old-rt"><span><i></i><b>&quot;RT&quot;</b></span></a>')
             .insertAfter( $(this).find('a.reply-action') )
             .click( function(ev){
                 rt( self );
                 return false;
             } );
        });
    }
    catch( er ){
        alert('Error:\n'+er.message);
    }
})( window, document );

Instead of an interval to add the RT button to new tweets I use jQuery's live event handler.

I add the 'old-rt' button on mouseover using a selector that only selects tweets that haven't got the 'old-rt' button yet.