Considering that Mac OS was under 5% a year ago, being over 10% now is a huge increase. From what I remember, Mac has been udner 5% for several years now, so 2009 was really strong for them.
Not sure why anyone would think Google’s market share was slipping. Must be all of those Bing ads. Google only continues to gain market share despite MS best efforts.
Also not sure how reliable StatCounter is, we used to use them until Google Analytics came out, so I’d imagine that’s the same story for many other websites. Most of the industry uses Hitwise Data as the most reliable source. Google is the only search engine increasing their market share according to their data.