index.js

$(document).ready(function(){
    if(!("WebSocket" in window)) {
      alert("Sorry, the build of your browser does not support WebSockets. Please use latest Chrome or Webkit nightly");
      return;
    }
    
    ws = new WebSocket("ws://localhost:8080/");
    ws.onmessage = function(evt) { 
      data = eval("(" + evt.data + ")");
      var p = $("<div class='tweet' style='display:none'><div class='content'><a class='main-screenname' href='http://www.twitter.com/" + data.user.screen_name + "/status/" + data.id + "' target='_blank'>" + data.user.screen_name + "</a> " + data.text + "</div></div>");
      if($('#tweets div.tweet').size() > 15) {
        $('#tweets div.tweet:last').slideDown(100, function() {
          $(this).remove();
        });
      }
      $('#tweets').prepend(p);
      p.slideDown(140);
    };
  });

may i suggest, that you replace eval with JSON.parse.
unless i am missing something there is no reason to use eval

yes you are totally right, thank you!