<p>When an <code>AutoCompleteExtender</code> is displayed in IE6 it seems to ignore z-index and renders below any select controls (like <strong>dropdownlists</strong>) in IE6.</p>

<pre><code>&lt;asp:TextBox ID="TextBox1" runat="server"  /&gt;
&lt;cc1:AutoCompleteExtender ID="AutoCompleteExtender1" runat="server" 
    TargetControlID="TextBox1" EnableCaching="true" CompletionSetCount="5" 
    FirstRowSelected="true" ServicePath="~/Services/Service1.asmx" ServiceMethod="GetSuggestion" /&gt;
&lt;asp:DropDownList ID="DropDownList1" runat="server"&gt;
    &lt;asp:ListItem Text="Item 1" Value="0" /&gt;
    &lt;asp:ListItem Text="Item 2" Value="1" /&gt;
&lt;/asp:DropDownList&gt;
</code></pre>

<p>How do I make it render above <strong>dropdownlists</strong>?</p>

Nothing renders below select controls in IE6. It's one of the many "features" microsoft bestowed upon us when they gifted IE to the world
You have to hide them, then re-show them.
Observe the
standard lightbox script
- which does exactly this
(note that link is just to the first thing I found on google which had the source to lightbox.js as a demonstration. It's got nothing to do with anything else)