The letter-spacing is treated differently on OSX than Windows XP. This can cause problems if you basing a design off text-based CSS. This fix works perfectly if you using a simple dropdown menu that uses absolute positions and auto margins, so its not a child of the initial menu you made, an example can be seen here: www.cmcri.org

A simple javascript code can detect a MAC platform with an IF statement, IF it is MAC then write to document a style. This code must be inserted after your initial call for your style sheet so it overwrites the original CSS code.

if(navigator.userAgent.indexOf('Mac') != -1)

{document.write ('<style type="text/css">#header ul li a{letter-spacing:.025em;}</style>');}

Easy fix!