i use RequiredFieldValidator to validate radtextbox,SetFocusOnError="true"
telerik version:2012Q3

first ，enter " "，validate event fire, display “notnull”
sencond，enter “123” ,radtextbox not set value，validate event not fire

asp.net textbox is ok

code：
<telerik:RadTextBox ID="txtAppId" runat="server" Width="300" AutoPostBack="false">
        </telerik:RadTextBox>
        <%--<asp:TextBox ID="txtAppId" runat="server" Width="300" AutoPostBack="false">
        </asp:TextBox>--%>
        <asp:RequiredFieldValidator ID="TextBoxRequiredFieldValidator" runat="server" Display="Dynamic"
            ControlToValidate="txtAppId" Text="NOT NULL" ErrorMessage="NOT NULL" SetFocusOnError="true">                          
        </asp:RequiredFieldValidator>