I have tested this code now with the Safari 5.1 and it is working fine. The normal menu shows till 768px wide, and then turns into the black dropdown one.
Have you tried clearing your browser’s cache? (http://www.refreshyourcache.com/en/safari-5/)
It should work, I have provided you a screenshot. It works for me in Safari, Firefox and Chrome.
There is no any js code which is hiding the default menu, this is just a css code which had display: none and block switching over two divs based on screen width.