In asp.net its easy to implement rounded corner to panels using ajax, try the following example
<%@ Page Language="C#" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server"></script>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<ajaxToolkit:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
</ajaxToolkit:ToolkitScriptManager>
</div>
<asp:Panel ID="Panel1" runat="server" BackColor="AppWorkspace" Height="188px" Width="271px">
</asp:Panel>
<ajaxToolkit:RoundedCornersExtender ID="Panel1_RoundedCornersExtender"
runat="server" Enabled="True" TargetControlID="Panel1" Radius="15">
</ajaxToolkit:RoundedCornersExtender>
</form>
</body>
</html>