Customise main-menu link - adding a '/' between  items
I would suggest a purely CSS approach to this. There is really no need to go through the trouble of adding a function to your theme for this change.

The following CSS code would get you (at least close to) the results you desire:

  ul.main-menu li:after {
    content: "/";
  }
  ul.main-menu li.last:after {
    content:"";
  }


The :after pseudo-class's support includes all browsers except IE 7 &amp; below and Firefox 3.0 and lower.