I thing that i found a bug. Tabstrip not working with asp:login together becouse tabstip fire validators in asp:login.
Code sample:
    
                                Code sample:
| <%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm1.aspx.cs" Inherits="ITDT.Gossip.Portal.WebForm1" %> | 
| <%@ Register TagPrefix="telerik" Namespace="Telerik.Web.UI" Assembly="Telerik.Web.UI" %> | 
| <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> | 
| <html xmlns="http://www.w3.org/1999/xhtml"> | 
| <head runat="server"> | 
| <title>Untitled Page</title> | 
| </head> | 
| <body> | 
| <form id="form1" runat="server"> | 
| <telerik:RadScriptManager ID="ScriptManager" runat="server" /> | 
| <div> | 
| <asp:Login runat="server"></asp:Login> | 
| <telerik:RadTabStrip ID="RadTabStrip1" runat="server"> | 
| <Tabs> | 
| <telerik:RadTab runat="server" Text="Tab1" PerTabScrolling="True" ScrollChildren="True" | 
| Selected="True"> | 
| <Tabs> | 
| <telerik:RadTab runat="server" Text="Child Tab 1" /> | 
| <telerik:RadTab runat="server" Text="Child Tab 2" Selected="True" /> | 
| <telerik:RadTab runat="server" Text="Child Tab 3" /> | 
| <telerik:RadTab runat="server" Text="Child Tab 4" /> | 
| <telerik:RadTab runat="server" Text="Child Tab 5" /> | 
| </Tabs> | 
| </telerik:RadTab> | 
| <telerik:RadTab runat="server" Text="Tab2" /> | 
| <telerik:RadTab runat="server" Text="Tab3" /> | 
| </Tabs> | 
| </telerik:RadTabStrip> | 
| </div> | 
| </form> | 
| </body> | 
| </html> |