OK I found the reason why somteimes DatePicker renders wrong in IE 9. It's the issue described in
http://blogs.telerik.com/aspnetmvcteam/posts/10-05-03/internet-explorer-css-limits.aspx.Using many RadControls on one page and DatePicker is on the end of the page the maximum of 31 stylesheets could by reached. Perhaps, the code of the RadContrls "fork" to several stylesheets in case of IE8 and IE9. The code below rendes correct in IE8 but in IE9 you see the DatePicker rendered wrong (There is no sense to use the controls in the way but only for demonstrating the issue)
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm1.aspx.cs" Inherits="RadDatePickerRenderProblem.WebForm1" %>
<head runat="server">
<title></title>
<!--[if IE 9]>
<link href="css/ie9fix1.css" rel="stylesheet" type="text/css" />
<link href="css/ie9fix2.css" rel="stylesheet" type="text/css" />
<![endif]-->
</head>
<body>
<form id="form1" runat="server">
<ajax:ToolkitScriptManager ID="RadScriptManager_1" runat="server" EnablePageMethods="true"
AsyncPostBackTimeout="600" />
<telerik:RadInputManager ID="RadInputManager1" runat="server">
<telerik:TextBoxSetting BehaviorID="TextBoxBehaviorOptional" EmptyMessage="">
<TargetControls>
</TargetControls>
</telerik:TextBoxSetting>
</telerik:RadInputManager>
<telerik:RadAjaxManager runat="server" ID="RadAjaxManager_1">
<AjaxSettings>
<telerik:AjaxSetting AjaxControlID="Settings1">
<UpdatedControls>
<telerik:AjaxUpdatedControl ControlID="Settings1" LoadingPanelID="RadAjaxLoadingPanel" />
</UpdatedControls>
</telerik:AjaxSetting>
</AjaxSettings>
</telerik:RadAjaxManager>
<telerik:RadAjaxLoadingPanel ID="RadAjaxLoadingPanel" runat="server" InitialDelayTime="200">
<div class="RadAjaxLoadingPanelText">
<div>
Loading...</div>
</div>
</telerik:RadAjaxLoadingPanel>
<div id="Settings1" runat="server" style="margin: 5px 5px 5px 5px; overflow: visible;">
<telerik:RadComboBox ID="RadComboBox1" runat="server" />
<telerik:RadPanelBar runat="server" ID="RadPanelBarExportSettings" ExpandMode="FullExpandedItem"
CssClass="PanelBar Big">
<Items>
<telerik:RadPanelItem Expanded="True" Text="PanelItem1" runat="server">
<Items>
<telerik:RadPanelItem Value="VersichertenInformationen" runat="server">
<ItemTemplate>
<telerik:RadListView ID="RadListView1" runat="server" EnableEmbeddedSkins="false"
EnableEmbeddedBaseStylesheet="false" />
<table border="0" cellpadding="0" cellspacing="0">
<tr>
<td>
<telerik:RadButton ID="RadButton1" runat="server" Text="RadButton">
</telerik:RadButton>
</td>
</tr>
<tr>
<td>
<telerik:RadRibbonBar ID="RadRibbonBar1" runat="server">
</telerik:RadRibbonBar>
</td>
</tr>
<tr>
<td>
<telerik:RadTreeView ID="RadTreeView1" runat="server">
</telerik:RadTreeView>
</td>
</tr>
<tr>
<td>
<telerik:RadMenu ID="RadMenu1" runat="server">
</telerik:RadMenu>
</td>
</tr>
<tr>
<td>
<telerik:RadSiteMap ID="RadSiteMap1" runat="server">
</telerik:RadSiteMap>
</td>
</tr>
<tr>
<td>
<telerik:RadMultiPage ID="RadMultiPage1" runat="server">
<telerik:RadPageView ID="RadPageView1" runat="server">
</telerik:RadPageView>
</telerik:RadMultiPage>
</td>
</tr>
<tr>
<td>
<telerik:RadTabStrip ID="RadTabStrip1" runat="server">
</telerik:RadTabStrip>
</td>
</tr>
<tr>
<td>
<telerik:RadToolBar ID="RadToolBar1" runat="server">
</telerik:RadToolBar>
</td>
</tr>
<tr>
<td>
<telerik:RadColorPicker ID="RadColorPicker1" runat="server">
</telerik:RadColorPicker>
</td>
</tr>
<tr>
<td>
<telerik:RadTagCloud ID="RadTagCloud1" runat="server">
</telerik:RadTagCloud>
</td>
</tr>
<tr>
<td>
</td>
</tr>
<tr>
<td>
<telerik:RadDatePicker runat="server" ID="txtGeburtsdatum" MinDate="01.01.1900" />
</td>
</tr>
</table>
</ItemTemplate>
</telerik:RadPanelItem>
</Items>
</telerik:RadPanelItem>
</Items>
</telerik:RadPanelBar>
</div>
</form>
</body>
</html>
The attached sceenshots show the pgae in IE8 and IE9.
All the best,
Thomas