Arriving fashionably late to the gathering, but if you can avoid catering to IE versions below 9, you can achieve this using purely CSS.
<div id="center">
<div id="tl"></div>
<div id="tr"></div>
<div id="bl"></div>
<div id="br"></div>
</div>
div {
width: 50px;
height: 50px;
background-color: #ccc;
border-radius: 100px;
}
#center {
position: relative;
margin: 100px 0 0 100px;
border: solid #fff 1px;
}
#tl, #tr, #bl, #br {
position: absolute;
height: 75px;
width: 75px;
z-index: -1;
}
#tl {
border-radius: 100px 0 0 0;
top: -50px;
left: -50px;
}
#tr {
border-radius: 0 100px 0 0;
top: -50px;
left: 26px;
}
#bl {
border-radius: 0 0 0 100px;
top: 26px;
left: -50px;
}
#br {
border-radius: 0 0 100px 0;
top: 26px;
left: 26px;
}
Experience the code in action here: http://jsfiddle.net/nchtG/
If supporting IE versions before 9 is necessary, as Andrew previously mentioned, Raphaël is your best bet.