You have done a great job regarding that. Never thought it will be that good in IE6.
Now I think there are many admins who will be glad to use XF without worrying about IE6.
the minimalistic approach helps in developing a style that is cross browser/version compatible
problems will arise when one adds their own customization, addons, etc
here's something to think about...
IE users are more likely to click on ads; if 10% of your users are on IE6 you can sure bet that you are losing 10% of your ad revenue
Not many places are supporting IE6 in any shape or form, that version is pretty much dead. Have had a few complaints that my site doesn't work in IE6, seems to be an issue with multiple float areas on pages, answer to those complaints is upgrade to a later version of IE or use an alternative browser like FF or Chrome.