Observe the below video, demonstrating the issue :

http://screencast.com/t/pC1V29UwYQ83 

Adding another, not visible textarea to absorb the focus can be used as a workaround:

<telerik:RadTabStrip ID="rtsRFT" runat="server" Skin="Office2007" MultiPageID="rmpRTF"
    SelectedIndex="0" ShowBaseLine="True" OnClientTabSelected="OnClientTabSelected">
    <Tabs>
        <telerik:RadTab Text="Description" TabIndex="61" ToolTip="Click to view description"
            SelectedCssClass="SelectedTab" PageViewID="pDesription">
        </telerik:RadTab>
        <telerik:RadTab Text="Notes" TabIndex="63" ToolTip="Click to view notes" PageViewID="pNotes">
        </telerik:RadTab>
    </Tabs>
</telerik:RadTabStrip>
<div class="welcomeTab" style="height: 100%">
    <telerik:RadMultiPage ID="rmpRTF" runat="server" SelectedIndex="0">
        <telerik:RadPageView ID="pDesription" runat="server">
            <div>
                <telerik:RadEditor ID="txtDescription" runat="server" Width="100%" Height="250px">
                </telerik:RadEditor>
            </div>
        </telerik:RadPageView>
        <telerik:RadPageView ID="pNotes" runat="server" Height="100%">
            <div>
                <telerik:RadEditor ID="txtNote" runat="server" Width="100%" Height="250px">
                </telerik:RadEditor>
            </div>
        </telerik:RadPageView>
    </telerik:RadMultiPage>

</div>

<script type="text/javascript">
    function OnClientTabSelected(sender, args) {
        $telerik.$("#inboxId").focus();
    }
</script>