I have been monitoring this trend for a couple of weeks to see if it remained constant, and it appears to be a real thing. Since the end of May 2009, Internet Explorer 7's browser share in the US has collapsed, with a requisite increase in the use of Firefox 3.0.
This is a staggering change. Either this is an artifact of the way that StatCounter is capturing browser data or a very large organization(s) suddenly switched the default browser that it allowed its customers to use.
Does anyone have any insight into why this may have occurred?