Not sure if this is a bug or not - might just be more of a compatibility issue: Use of ASP.NET AJAX and ExtJS's quick tips together seems to cause large memory problems in IE 7. A simple .aspx page to reproduce:
<%@ Page Title="" Language="C#" AutoEventWireup="false" 
  CodeFile="LeakTest.aspx.cs" Inherits="LeakTest" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "">

<html xmlns="">
    <head id="Head1" runat="server">
        <title>Untitled Page</title>
        <link rel="stylesheet" type="text/css" href="scripts/ext/resources/css/ext-all.css" />

        <script type="text/javascript" src="scripts/ext/adapter/ext/ext-base.js"></script>
        <script type="text/javascript" src="scripts/ext/ext-all-debug.js"></script>
        <form id="mainForm" runat="server">
            <asp:ScriptManager ID="scriptManager" runat="server" />
            <script type="text/javascript">
              Ext.onReady(function() {
Running the page in sIEve will show IE leaking ~1MB every page refresh. FF 3 seems to reclaim memory fine. Note that removing either the ScriptManager tag or the Ext.QuickTips.init statement will stop the leaking. Any other ExtJS code besides initializing QuickTips seems irrelavant to the problem. Also, using a global function for Ext.onReady instead of an anonymous function doesn't affect the problem either.

I traced through the Ext.QuickTips.init code (and code it calls) but couldn't find anything suspicious, maybe someone could provide some perspective on how this could occur?