I haven't read through the entire thread but I haven't seen this mentioned.
Google's Code contest produced this fairly simple script.
You can allow users to continue to use your site or you can edit the JS file and remove the bypass from the popup forcing them to upgrade or switch to another browser like Fire Fox or Opera to view your forums.
http://www.downloadsquad.com/2009/03/20/web-developers-help-rid-the-world-of-ie6-with-a-simple-script/
http://code.google.com/p/sevenup/