Since Yahoo was at #1 through the strength of their social media and not their search engine, I wonder how Google took over - they don't have that kind of draw at all. Maybe it's increased traffic to Google's webmaster help pages? ;)
In terms of reach, according to the Alexa data, they've been beating Yahoo! for over a year. In terms of ranking, they've been #1 since mid-January. I was going to say the launch of Chrome might have something to do with it, but I don't think it's even possible to use the Alexa toolbar in that browser (besides the fact that I can't imagine why anyone would want to). Plenty of other services have been launched or updated throughout the last year or so. Think of Google Sites, new releases of software, language expansions, updates to Maps and Translate, etc. However, the most important factor will, no doubt, be that they still have the best search engine on the web.