Here are a list of unpatched or "partially fixed" bugs in IE6 according to Secunia.com. Some of these date back to 2003. Though some of considered only slightly problematic or slight chance and hence seems like they were not patched, some of the bugs in this list could cause a lot of problems if they happen to a particular user.
Additionally I found it interesting that Secunia lists a higher percentage of "advisories" unpatched in IE7. However going through the whole list of items for IE7 there are only 9 unpatched items compared to over 20 below. I would conclude based on that, the traffic I have mentioned in previous posts that looks a bit odd, and just by comparing the different types of bugs outstanding that IE7 is a much better browser choice if concerned about security.
Unpatched IE6 Bug - FTP Injection
IE 6 may disclose sensitive information with OnKeyDown event
Printing table of links from IE6 or IE7
IE6 and IE7 FTP credentials exposure
IE6 allows faking a URL in the address bar
Internet Explorer 6 or 7 File Upload Form Keystroke Event Cancel Vulnerability
IE6 hidden network share weakness
IE5 and IE6 Drag and Drop Vulnerability
IE6 - Trick a use to go to a malicious site
IE6 - XMLHTTP HTTP Request Injection
IE6 - Microsoft Internet Explorer Dialog Origin Spoofing Vulnerability
IE6 Microsoft Internet Explorer Popup Title Bar Spoofing Weakness
IE6 Internet Explorer Global Variables Local File Detection Weakness
IE6 - Window Injection Vulernability
IE6 - save as picture download spoofing - trick users into downloading malicious files
IE6 - cookie vulnerability
Note this bug says partially fixed.
IE6 - bypass file download security warning and save as displays different file extension than actual
Internet Explorer Flash/Excel Content Status Bar Spoofing Weakness
IE6 - Detect the presence of local files
(partial fix)
IE6 - cross domain cookie vulnerability
IE5 & IE6 address bar - faking urls
(partial fix)
IE6 - create popup content overlay
(partial fix)
IE6 bug - fake urls (partial fix)
IE6 - Cross frame scripting restriction bypass
IE6 - Internet Explorer File Identification Variant
IE6 - Exposure of Installed Components
IE6 - Internet Explorer Custom HTTP Error Script Injection Vulnerability (partial fix)
IE6 - Exposes sensitive information (partial fix)