Hello,

Would appear as of revision 800 that ViewState has died, check out the following simple example:

Example.aspx:
<%@ Page Language="C#" %>
<%@ Register assembly="Coolite.Ext.Web" namespace="Coolite.Ext.Web" tagprefix="ext" %>

<!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 id="Head1" runat="server">
    <title>ComboBox PostBacks - Coolite Toolkit Example</title>
    <script runat="server">

     </script>
</head>
<body>
    <p><a href="Postbacks.aspx">Reload</a></p>
    <form id="form1" runat="server">
        <asp:ScriptManager ID="ScriptManager1" runat="server" EnablePartialRendering="True" />
        <ext:ScriptManager ID="ScriptManager2" runat="server" />
        <ext:TabPanel ID="TabPanel1" runat="server" AutoPostBack="True" ActiveTabIndex="0" DeferredRender="True">
            <Tabs>
                <ext:Tab ID="Tab1" runat="server">
                    <Body>
                        <ext:TextField ID="TextField1" runat="server" />
                    </Body>
                </ext:Tab>
                <ext:Tab ID="Tab2" runat="server">
                    <Body>
                        <ext:TextField ID="TextField2" runat="server" />
                    </Body>
                </ext:Tab>
            </Tabs>
        </ext:TabPanel>
    </form>
</body>
</html>
Replication steps:

1. Load page
2. Type into TextField1
3. Switch to Tab 2
4. Type into TextField2
5. Switch back to Tab 1 everything is gone

Cheers,
Timothy