Tabbing to decorated select element by RadFormDecorator, focuses the original select element. For the time being the following workaround can be used:

JavaScript:

        <script>
            function pageLoad() {
                $ = $telerik.$;
                $.each($(".rfdRealInput"), function (index, value) { value.setAttribute("tabindex", "-1") });
            }
        </script>

ASPX:
        <telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="All" RenderMode="Lightweight" />
        <asp:TextBox ID="Textbox1" runat="server" />
        <asp:DropDownList ID="Dropdownlist1" runat="server">
            <asp:ListItem Text="text1" />
            <asp:ListItem Text="text2" />
        </asp:DropDownList>
        <asp:TextBox ID="Textbox2" runat="server" />
        <telerik:RadButton ID="RadButton1" runat="server" />