<form runat="server">
<asp:RadioButtonList id=RadioButtonList1 runat="server">
<asp:ListItem>Item 1</asp:ListItem>
<asp:ListItem>Item 2</asp:ListItem>
<asp:ListItem>Item 3</asp:ListItem>
<asp:ListItem>Item 4</asp:ListItem>
<asp:ListItem>Item 5</asp:ListItem>
<asp:ListItem>Item 6</asp:ListItem>
</asp:RadioButtonList>

<input type="button" value="Значение" onclick="getRadioListValue(this.form, 'RadioButtonList1')">
</form>

<script language="javascript" type="text/javascript">
function getRadioListValue(f, listName)
{
    var list = f.elements[listName];
    for (var i=0; i<list.length; i++)
    {
        if (list[i].checked)
        {
            alert(list[i].value);
            return;
        }
    }
    alert("Ничего не выбрано!");
}
</script>