$(document).ready(distributeMenu);

function distributeMenu() {
   var menuItems, itemsWidth, menubarWidth, menuSpacing;

   menuItems = $('.mainmenu_item');
   itemsWidth = 0;
   menubarWidth = $('#menu').width();

   menuItems.each(function () {
      itemsWidth += this.offsetWidth;
   });

   menuSpacing = Math.floor((menubarWidth - itemsWidth) / (menuItems.length-1));

   menuItems.each(function (i) {
      if (i == menuItems.length - 1) {
         return;
      }
      this.style.marginRight = menuSpacing + 'px';
   });
}