"We can look forward to a fascinating battle between Microsoft and Google as the pace of growth of Chrome suggests that it will become a real rival to Internet Explorer globally," commented Aodhan Cullen, CEO, StatCounter. "Our stats measure actual browser usage, not downloads, so while Chrome has been highly effective in ensuring downloads our stats show that people are actually using it to access the web also."

http://gs.statcounter.com/press/chrome-overtakes-firefox-globally-for-first-time

I heard some people claiming that StatCounter was not the most reliable source. I don't see Chrome leading FF on any of my sites yet, but it's close on some.