Has anyone from Fusion Charts weighed in on this yet?  I am having the same issue.   
I am using the setDataURL method.  I have tested this where I added a sleep command to my data XML page to intentionally simulate a slow data page.  On my chart page, the browser freezes until the XML is returned to the chart.   
I am using Firefox 3.0.10 and IE 7. 
It seems like the call for the XML is blocking other processes.  Is there a way to make this call asyncronous so as NOT to lock the browser?