I’m having the same problem exactly. But when I copy and paste this code:
#header #mainmenu ul li:last-child { border-right: none; }
#header #mainmenu ul li:last-child a { padding: 0 15px; }
which burak says worked for him –it doesn’t work for me.
The right hand end of my menu has white space when you hover over “Contact Us”
My url : http://arnoldgenealogy.com