A nice WebKit-only solution that takes advantage of the background-clip: text support: http://jsfiddle.net/sandro_paganotti/wLkVt/
span{
font-size: 100px;
background: linear-gradient(to right, black, black 50%, grey 50%, grey);
-webkit-background-clip: text;
-webkit-text-fill-color: transparent;
}