I needed something similar, and came up with using the :before (or :after) pseudoclasses:
#mydiv {
background-color: #fbb;
margin-top: 100px;
position: relative;
}
#mydiv:before {
content: "";
background-color: #bfb;
top: -100px;
height: 100px;
width: 100%;
position: absolute;
}