Well, my first step was to roll-back to the previous Joomla template (different html layout/different css) we were using and that got rid of the error in IE6/7!
Now I will try to figure out whether the issue is in the html tag layout (per the Microsoft KB article above) or whether there is a javascript conflict/difference between the working and non-working templates.
Because you are running Joomla 1.5 I can't say that our problems are the same but you might try briefly assigning a different default template and seeing if your site successfully loads in IE6/7.
I hope someone more familiar with FC can chime in about if/how the IE6/7 bug detailed above relates to FusionCharts javascript processing...
UPDATE:
I haven't had much time to play with this but my case is definitely one of javascript conflict. It is either too much javascript loading for IE's poor little brain or it is a conflict among the scripts that are loading. If I comment out several scripts that are loading but are not being used right now then everything works fine. I made one attempt to change my mootools.js and that gave mixed results...sometimes the site would crash in IE and sometimes not. I'm going to keep trying to narrow this down because I don't want IE to tie my hands in this way!