It's just using two box shadows, one inset and the other outset, i.e:
.box {
width: 100px;
height: 100px;
box-shadow: 0 3px 6px rgba(0,0,0,0.3), inset 0 -3px 3px rgba(0,0,0,0.1);
border: solid #ccc 1px;
border-radius: 10px;
margin: 50px 0 0 50px;
}
See it here: http://jsfiddle.net/WYLJv/