try this:
<head runat="server">
<style type="text/css">
.txtbox
{
border-top-left-radius: 20px;
border-top-right-radius: 20px;
border-bottom-left-radius: 20px;
border-bottom-right-radius: 20px;
}
</style>
</head>
and apply the css class to textbox as:
<asp:TextBox ID="TextBox1" runat="server" cssclass="txtbox"></asp:TextBox>
this definitely works.