For the time being you can capture the keypress event of the submitting button and cancel it only if the Enter key is pressed. For example:

        <telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
            <script>
                Sys.Application.add_load(function () {
                    var decoratedButton = Telerik.Web.UI.RadFormDecorator.getDecoratedElement($get("<%=SearchButton.ClientID%>"));
                    cancelKeyPress(decoratedButton);
                });

                function cancelKeyPress() {
                    Array.forEach(arguments, function (element) {
                        $addHandler(element, "keypress", function (event) {
                            if (event.charCode == 13)
                                $telerik.cancelRawEvent(event.rawEvent);
                        });
                    });
                }

            </script>
        </telerik:RadCodeBlock>
        <telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="All" />
        <asp:Panel ID="SearchPanel" runat="server" DefaultButton="SearchButton">
            <telerik:RadTextBox FocusedStyle-CssClass="textfocused" TabIndex="1" HoveredStyle-CssClass="texthover"
                ID="SearchText" runat="server" Width="200px" Height="22px" />
            <asp:Button ID="SearchButton" runat="server" Text="Search" TabIndex="3" OnClick="SearchButton_Click" OnClientClick="alert(1);return true;" />
        </asp:Panel>