This was really bugging me and I guess others as well.
In the forums they thought it was a bug in IE6 (http://forums.asp.net/thread/1488886.aspx), but I was getting the same behavior in IE6, and Firefox
Setting the Height in the content panel (the collapsable area) to 0 solved the problem for me in IE and Firefox.
<asp:Panel ID="ContentPanel" runat="server"
CssClass="collapsePanel" Height="0">
Anyone who still has IE6 installed, let me kow if you still get the panel “flash” on page load.