This might seem obvious but it had me stumped for a while. Most ASP.NET programmers should know that if you set EnableViewState to false on a DropDownList, the items that were bound to that control will not be persisted on postback. What you might forget (as I did) is that if your DropDownList (or any other DataBound control for that matter) is held within another control (such as a Panel) that has view state disabled, then it also applies to anything contained within that control.
I added a DevExpress Panel to my MasterPage surrounding a ContentHolder, then wondered why none of my DropDownLists were working correctly.
Mental note for me to remeber this one for the future.
Example
Here is an example to illustrate the issue and the solution:
ASPX Page
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="WebApplication1._Default" %>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>DropDownList ViewState Example</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:Panel ID="Panel1" runat="server" EnableViewState="false">
<asp:DropDownList ID="DropDownList1" runat="server">
<asp:ListItem Text="Item 1" Value="1"></asp:ListItem>
<asp:ListItem Text="Item 2" Value="2"></asp:ListItem>
<asp:ListItem Text="Item 3" Value="3"></asp:ListItem>
</asp:DropDownList>
</asp:Panel>
<asp:Button ID="Button1" runat="server" Text="Submit" OnClick="Button1_Click" />
</div>
</form>
</body>
</html>