OK, I got it working as a standalone test. It was not working when the sample code I posted was part of a larger page that was within another callback panel, at least not until I changed the UpdateMode for that parent callback panel to "Conditional".
HOWEVER - I have a new issue. I am now trying a simpler setup with just a single callback panel for the entire page except a navigation menu. No matter what I try, the contents within the callback panel are "squished together" and a flash object that is part of the contents is not being displayed at all. I tried changing all sorts of settings like the UpdateMode of the callback panel, the height, etc. Nothing changes it. If I use an AJAX toolkit Update Panel in place of the EssentialObjects callback panel, it works fine - but I would much prefer to use the EssentialObjects one. What the heck is going on??? Why is everything squished together? Here is a simplified version of my code:
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="test.aspx.cs" Inherits="_admin_test" %>
<%@ Register Assembly="EO.Web" Namespace="EO.Web" TagPrefix="eo" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server">
public string DocID = "";
public string AccessKey = "";
public string DocFileName = "";
</script>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<div style="width: 100%; border-top: #e0e0e0 1px solid; border-bottom: #e0e0e0 1px solid;
background-image: url('/_images/menubg.gif');">
menu here...
</div>
<eo:CallbackPanel ID="CallbackPanel1" runat="server" Width="100%" LoadingHTML="<table cellpadding='10'><tr><td><img src='/_images/loading.gif'></td><td>Loading...</td></tr></table>"
AutoDisableContents="False" UpdateMode="Group">
<eo:AJAXUploader AutoUpload="true" runat="server" ID="AJAXUploader1" MaxDataSize="30000"
AutoPostBack="true" TempFileLocation="~/temp">
<LayoutTemplate>
<table cellspacing="0" cellpadding="2" width="520" border="0">
<tr id="postedfilesarea" runat="server" visible="false">
<td colspan="3">
<asp:PlaceHolder ID="PostedFilesPlaceHolder" runat="server">Posted Files Place Holder
</asp:PlaceHolder>
<asp:Button ID="DeleteButton" runat="server" Text="Delete Selected Files"></asp:Button>
<p>
</p>
</td>
</tr>
<tr>
<td width="99%">
<asp:PlaceHolder ID="InputPlaceHolder" runat="server">Input Box Place Holder</asp:PlaceHolder>
</td>
<td>
<asp:Button ID="UploadButton" runat="server" Text="Upload"></asp:Button>
</td>
<td>
<eo:ProgressBar ID="ProgressBar" runat="server" ControlSkinID="None" Height="18px"
Width="150px" BorderColor="#336699" BorderStyle="Solid" BorderWidth="1px" IndicatorColor="151, 198, 232">
</eo:ProgressBar>
</td>
</tr>
</table>
</LayoutTemplate>
</eo:AJAXUploader>
<table width="100%" cellpadding="0" cellspacing="0">
<tr>
<td style="padding-top: 2px; padding-bottom: 2px; padding-left: 40px; border-top: #C0C0C0 0px solid;">
<asp:DropDownList ID="ddlMenu" runat="server" AutoPostBack="true">
</asp:DropDownList>
</td>
<td align="left" style="padding-top: 2px; padding-bottom: 2px; border-top: #C0C0C0 0px solid;
padding-right: 4px; padding-right: 4px;">
<asp:HyperLink ID="HyperLink1" runat="server" Target="_blank" Text="Open as PDF (large file size)"></asp:HyperLink>
</td>
<td align="left" style="padding-top: 2px; padding-bottom: 2px; border-top: #C0C0C0 0px solid;
padding-right: 4px; padding-right: 4px;">
<asp:LinkButton ID="lbDeleteNewsletter" runat="server"></asp:LinkButton>
</td>
</tr>
</table>
<object codebase="http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=9,0,0,0"
id="-671065188" name="-671065188" classid="clsid:d27cdb6e-ae6d-11cf-96b8-444553540000"
align="middle" height="600" width="798">
<param name="movie" value="http://documents.scribd.com/ScribdViewer.swf?document_id=<%= DocID %>&access_key=<%= AccessKey %>&page=1&version=1&mode=tile">
<param name="quality" value="high">
<param name="play" value="true">
<param name="loop" value="true">
<param name="scale" value="showall">
<param name="wmode" value="opaque">
<param name="devicefont" value="false">
<param name="bgcolor" value="#ffffff">
<param name="menu" value="true">
<param name="allowFullScreen" value="true">
<param name="allowScriptAccess" value="always">
<param name="salign" value="">
<embed src="http://documents.scribd.com/ScribdViewer.swf?document_id=<%= DocID %>&access_key=<%= AccessKey %>&page=1&version=1&mode=tile"
quality="high" pluginspage="http://www.macromedia.com/go/getflashplayer" play="true"
loop="true" scale="showall" wmode="opaque" devicefont="false" bgcolor="#ffffff"
name="-671065188_object" menu="true" allowfullscreen="true" allowscriptaccess="always"
salign="" type="application/x-shockwave-flash" align="middle" height="600" width="798"></embed>
</object>
</eo:CallbackPanel>
</div>
</form>
</body>
</html>