gistfile1.js

// allow $('elem').css({ 'left' : '+=10px' });

// so you dont need to do:
//   $('elem').animate({ 'left' : '+=10px' }, 0);

// by paul irish
// for ralph holzmann
// http://github.com/paulirish/lazyweb-requests/issues#issue/10

(function($, oldcss){

  // magic from the core.
  var rfxnum = /^([+\-]=)?([\d+.\-]+)(.*)$/;

  $.fn.css = function(obj, val){
    
    
    var parts = rfxnum.exec(val), 
        that  = this;
    
    
    if ($.isPlainObject(obj)){
      $.each(obj, function(k, v){
        $(that).css(k, v);
      });
    
    // here's the magic.  
    } else if (val && parts && parts[1]) {
      
      var end = parseFloat(parts[2])
      
      return oldcss.call(this, obj, function(index, currentValue){
        return ((parts[1] === "-=" ? -1 : 1) * end) + parseFloat(currentValue);
      });
    
    // no fancypants  
    } else {
      
      return oldcss.apply(this, arguments);
    }
    
    return this;
    
  };

})(jQuery, jQuery.fn.css);

Why not use the callback function of the css method to increment values? Per the jQuery API example:
$('div.example').css('width', function(index) {
return index * 50;
});

That essentially what i'm doing. :)